Split TESTDIR into TESTLOGDIR and TESTDATADIR

Split TESTDIR into TESTLOGDIR and TESTDATADIR

The motivation for this is twofold. For one the meson patchset would like to
have more control over the logfiles. For another, the log file location for
tap tests (tmp_check/log) is not symmetric to the log location for
pg_regress/isolation tests (log/).

This commit does not change the default location for log files for tap tests,
as that'd break the buildfarm log collection, it just provides the
infrastructure for doing so.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/1131990.1660661896@sss.pgh.pa.us
Discussion: https://postgr.es/m/20220828170806.GN2342@telsasoft.com