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

This generates fewer error markers in syntax-aware editors.

indent: clean up the code, add a few tests

tests/indent: separate stdout from stderr

Due to buffering, there was no guarantee that the output would be
exactly in the same order as written in the tests.

indent: use correct preprocessing directive in error message

indent: allow more than 5 levels of #if/#endif

indent: preserve indentation of preprocessor directives

tests/indent: test interaction between preprocessing and comments

indent: don't touch comments in preprocessing lines

The indentation of multi-line comments was wrong, and the code for
handling them was too complicated.

tests/indent: add more tests for preprocessing directives

tests/indent: test multi-line comments in preprocessing lines

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: clean up and extend tests

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.