tests/make: replace 'variable expression' with 'expression'

Each expression is based on a variable, there's no need for the
verbosity. The wording in make's diagnostics will be changed in a
follow-up commit.

make: remove filler word 'Do' from function names for parsing

No functional change, except for debug logging.

make: in the test varname-empty, keep more debug output

Filtering out the interesting lines based on the very specific pattern
"Var_Set" was surprising.

make(1): add tests for the variable named ""

make(1): extend documentation in the varname-empty test

make(1): try to set the variable named "" using the command line

Before 2020-08-22, the command line argument '=cmline-plain' triggered
undefined behavior, just like the assignment '=assigned' in the Makefile.

make(1): fix undefined behavior when assigning to variable ""

Using a programming language with built-in array bounds checks would
have prevented this bug, and many others as well.

make(1): try a bit harder to set the variable with the empty name

There is not only the != assignment operator, but a few others as well.
Especially the += operator is interesting since it has a different code
path. But all is good.

make(1): prevent assignment to the variable with the empty name

make(1): add test for assigning to the variable named ""

This has been possible at least since 2013, probably much longer.

make: remove filler word 'Do' from function names for parsing

No functional change, except for debug logging.

make: in the test varname-empty, keep more debug output

Filtering out the interesting lines based on the very specific pattern
"Var_Set" was surprising.

make(1): add tests for the variable named ""

make(1): extend documentation in the varname-empty test

make(1): try to set the variable named "" using the command line

Before 2020-08-22, the command line argument '=cmline-plain' triggered
undefined behavior, just like the assignment '=assigned' in the Makefile.

make(1): fix undefined behavior when assigning to variable ""

Using a programming language with built-in array bounds checks would
have prevented this bug, and many others as well.

make(1): try a bit harder to set the variable with the empty name

There is not only the != assignment operator, but a few others as well.
Especially the += operator is interesting since it has a different code
path. But all is good.

make(1): prevent assignment to the variable with the empty name

make(1): add test for assigning to the variable named ""

This has been possible at least since 2013, probably much longer.

make: in the test varname-empty, keep more debug output

Filtering out the interesting lines based on the very specific pattern
"Var_Set" was surprising.

make(1): add tests for the variable named ""

make(1): extend documentation in the varname-empty test

make(1): try to set the variable named "" using the command line

Before 2020-08-22, the command line argument '=cmline-plain' triggered
undefined behavior, just like the assignment '=assigned' in the Makefile.

make(1): fix undefined behavior when assigning to variable ""

Using a programming language with built-in array bounds checks would
have prevented this bug, and many others as well.

make(1): try a bit harder to set the variable with the empty name

There is not only the != assignment operator, but a few others as well.
Especially the += operator is interesting since it has a different code
path. But all is good.

make(1): prevent assignment to the variable with the empty name

make(1): add test for assigning to the variable named ""

This has been possible at least since 2013, probably much longer.

make(1): add tests for the variable named ""

make(1): extend documentation in the varname-empty test

make(1): try to set the variable named "" using the command line

Before 2020-08-22, the command line argument '=cmline-plain' triggered
undefined behavior, just like the assignment '=assigned' in the Makefile.

make(1): fix undefined behavior when assigning to variable ""

Using a programming language with built-in array bounds checks would
have prevented this bug, and many others as well.

make(1): try a bit harder to set the variable with the empty name

There is not only the != assignment operator, but a few others as well.
Especially the += operator is interesting since it has a different code
path. But all is good.

make(1): prevent assignment to the variable with the empty name

make(1): add test for assigning to the variable named ""

This has been possible at least since 2013, probably much longer.