Support retaining data dirs on successful TAP tests

Support retaining data dirs on successful TAP tests

This moves the data directories from using temporary directories with
randomness in the directory name to a static name, to make it easier to
debug.  The data directory will be retained if tests fail or the test
code dies/exits with failure, and is automatically removed on the next
make check.

If the environment variable PG_TEST_NOCLEAN is defined, the data
directories will be retained regardless of test or exit status.

Author: Daniel Gustafsson <daniel@yesql.se>