Added tests for POSIX captures.

Added tests for POSIX captures.

Tests are adopted from Glenn Fowler's testregex suite and its derivative
regex-posix-unittest by Christopher Kuklewicz.

Irrelevant tests have been dropped:

    - tests for specific POSIX features not supported by RE2C, like
      begin/end markers '$' and '^', backreferences, predefined character
      classes (most of them come from 'basic' subdirectory)

    - all 'leftassoc' tests: they are exact copies of 'rightassoc' tests,
      differ only in match results and are supposed to fail

    - all tests from 'categorize' that are supposed to fail

I also added a few tests of my own (in the 'other' subdirectory).

Tests are arranged as a bunch of '.dat' files (each file contains lines of
the form '<regexp> <string> <match>' or blank lines) and two scripts.

    - '__run.sh' runs all tests (generates temporary '.re' file with the
      given regexp, compiles it, runs against the given string and checks
      that the result coincides with the given result)

    - '__gen.sh' regenerates '.re' files for regular RE2C test suite (the
      generated files do not contain any code, just the bare regexp)

All regexps are pre-patched to exclude '\x00' (terminating NULL) character
from subregexps like '.' or '[^a]: otherwise RE2C-generated programs would
greedily consume all characters and fail. Note that if only '.' was present
in the original tests, we might have used '\n' as terminator, but '[^a]'
includes newline. Also '()' subregexp was substituted with '("")'.