tests/make: demonstrate unhelpful debug log in case of errors

The releng build job that runs lint fails. It outputs the usage message
of lint, which doesn't include the failing option. After that, make
outputs:

*** Failed target: lint-cgdconfig
*** Failed commands:
${LINT} ${LINTFLAGS} \
${_LDFLAGS.${:Ucgdconfig}:C/-L[ ]*/-L/Wg:M-L*} \
${LOBJS.${:Ucgdconfig}} ${_LDADD.${:Ucgdconfig}}

Make doesn't output the expanded command, therefore the log doesn't show
the offending option '-pthread' that leads to the usage message.

tests/make: test the combination of -de with -j1

The test cases are the same as in opt-debug-errors.mk. The output
differs in several details though.

Even though the option '-k' is given (which is the default for any tests
that don't override it in unit-tests/Makefile), there is no message
"(continuing)" anywhere.

The failed target is printed twice. Once before the failed commands,
once after. This redundancy is not necessary and may be removed in a
follow-up commit.

The printed commands are in their unexpanded form, which may or may not
be more helpful than the expanded and space-normalized form of compat
mode. Either way, this is an unnecessary inconsistency between compat
mode and jobs mode.

In jobs mode, the message "make: stopped in $dir" is printed for each
failure, which is helpful since each of the jobs may have started in a
separate directory.

tests/make: test the combination of -de with -j1

The test cases are the same as in opt-debug-errors.mk. The output
differs in several details though.

Even though the option '-k' is given (which is the default for any tests
that don't override it in unit-tests/Makefile), there is no message
"(continuing)" anywhere.

The failed target is printed twice. Once before the failed commands,
once after. This redundancy is not necessary and may be removed in a
follow-up commit.

The printed commands are in their unexpanded form, which may or may not
be more helpful than the expanded and space-normalized form of compat
mode. Either way, this is an unnecessary inconsistency between compat
mode and jobs mode.

In jobs mode, the message "make: stopped in $dir" is printed for each
failure, which is helpful since each of the jobs may have started in a
separate directory.