tests/indent: format stderr output as end-of-line comments

This generates fewer error markers in syntax-aware editors.

indent: miscellaneous cleanups, more tests for edge cases

tests/indent: migrate token tests to other tests

In indent.h 1.49 from 2021-10-25, the enumeration token_type was split
into lexer_symbol and parser_symbol to more clearly express that these
tokens fall into completely different classes of usage patterns.

tests/indent: change directive from '#' to '//'

Using a '//' instead of '#' turns the directives into well-formed C
code, resulting in fewer error markers in the editor.

indent: remove FreeBSD IDs

Most of the IDs were empty anyway.

tests/indent: add skeletons for testing tokens and parser symbols

The constants that were previously defined in indent_codes.h were a wild
mixture of tokens from the lexer and symbols on the parser stack. They
were split into separate types starting at indent.h 1.49 from 2021-10-25
and finishing at 1.73 from 2021-10-31.

To match the tests with the new token names, the old tests need to be
migrated to the newly added tests. This will take some time so first
add the skeletons and migrate them in smaller steps, cleaning them up
and extending them on the way.

tests/indent: migrate token tests to other tests

In indent.h 1.49 from 2021-10-25, the enumeration token_type was split
into lexer_symbol and parser_symbol to more clearly express that these
tokens fall into completely different classes of usage patterns.

tests/indent: change directive from '#' to '//'

Using a '//' instead of '#' turns the directives into well-formed C
code, resulting in fewer error markers in the editor.

indent: remove FreeBSD IDs

Most of the IDs were empty anyway.

tests/indent: add skeletons for testing tokens and parser symbols

The constants that were previously defined in indent_codes.h were a wild
mixture of tokens from the lexer and symbols on the parser stack. They
were split into separate types starting at indent.h 1.49 from 2021-10-25
and finishing at 1.73 from 2021-10-31.

To match the tests with the new token names, the old tests need to be
migrated to the newly added tests. This will take some time so first
add the skeletons and migrate them in smaller steps, cleaning them up
and extending them on the way.