Fix in-tree `recheck' after failed test that runs `check'. If a test like pr401b-p.test fails for some reason, a `make recheck' would cause another reason for failure because it sets AM_MAKEFLAGS which gets propagated into the test and causes its `make check' to fail. * tests/defs.in: Also unset AM_MAKEFLAGS. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>