tests: Extend $TEST_FUNCTIONS with predefined functions for skipping tests etc.

Apply this change across all the shell scripts containing tests.

Additionally this defines the environment variables $abs_srcdir,
$abs_builddir, $top_srcdir, $top_builddir, $abs_top_srcdir and
$abs_top_builddir which can now be used throughout test scripts.

fish/test-a.sh

@@ -20,6 +20,9 @@
 
 set -e
 
+$TEST_FUNCTIONS
+skip_if_skipped
+
 rm -f test-a.out
 rm -f test-a.img