self checks: avoid spurious failures when keep_testdirs=yes * t/self-check-cleanup.tap, t/self-check-dir.tap: Unset the 'keep_testdirs' environment variable, to avoid spurious errors when the testsuite is run with "keep_testdirs=yes make check". Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>