* tests: Introduced a new flag for tests -- PGPROBACKUP_GDB
* tests: Do travis builds with CFLAGS="-O0" (stabilization of gdb tests)
* tests: Run compatibility tests only if PGPROBACKUPBIN_OLD set
* tests: Running some tests now depends on PGPROBACKUP_SSH_REMOTE
With autovacuum enabled tests are unstable. Especially they are unstable
if postgresql is running under valgrind and therefore is severely
inhibited (and vacuum has time to be triggered).
