tests: create an interactive gdb session with the 'debug' helper

tests: create an interactive gdb session with the 'debug' helper

The 'debug' test helper is supposed to facilitate debugging by running
a command of the test suite under gdb.  Unfortunately, its usefulness
is severely limited, because that gdb session is not interactive,
since the test's, and thus gdb's standard input is redirected from
/dev/null (for a good reason, see 781f76b15 (test-lib: redirect stdin
of tests, 2011-12-15)).

Redirect gdb's standard file descriptors from/to the test
environment's stdin, stdout and stderr in the 'debug' helper, thus
creating an interactive gdb session (even in non-verbose mode), which
is much, much more useful.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>