tests: make a test script more semantic This is mostly useful for Automake-NG, that is heavily overhauling the generated Makefiles and thus is prone to break grepping checks (which can sometimes end up causing false negatives in the testsuite, sadly). But this is not a reason not to strengthen the test for mainline Automake as well. * t/noinstdir.sh: Add semantic checks. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>