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>