Don't hardcode tmp_check/ as test directory for tap tests

Don't hardcode tmp_check/ as test directory for tap tests

This is motivated by the meson patchset, which wants to put the log / data for
tests in a different place than the autoconf build. Right now log files for
tap tests have to be inside $TESTDIR/tmp_check, whereas log files for
pg_regress/isolationtester are outside of tmp_check. This change doesn't fix
the latter, but is a prerequisite.

The only test that needs adjustment is 010_tab_completion.pl, as it hardcoded
the tmp_check/ directory. Instead create a dedicated directory for the test
files.  It's also a bit cleaner independently, because it doesn't intermingle
the test files with more important things like the log/ directory.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/1131990.1660661896@sss.pgh.pa.us
Discussion: https://postgr.es/m/d861493c-ed20-c251-7a89-7924f5197341@enterprisedb.com