test: offer "automake" output and check for perl better

runtests.pl -am now uses the "PASS/FAIL: [desc]" output for each
executed test. You can run 'make test-am' in the root build directory to
invoke that. The reason for this output style is to better allow generic
test suite parsers to also grok our test output.

The test Makefile now also tests that perl was indeed found and that the
PERL variable points to an executable before it tries to run the main
test perl script runtests.pl,