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.

format/test-virt-format-docs.sh

@@ -16,7 +16,9 @@
 # along with this program; if not, write to the Free Software
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
-export LANG=C
 set -e
 
-$srcdir/../podcheck.pl virt-format.pod virt-format
+$TEST_FUNCTIONS
+skip_if_skipped
+
+$top_srcdir/podcheck.pl virt-format.pod virt-format

format/test-virt-format.sh

@@ -20,10 +20,8 @@
 
 set -e
 
-if [ -n "$SKIP_TEST_VIRT_FORMAT_SH" ]; then
-    echo "$0: test skipped because environment variable is set."
-    exit 77
-fi
+$TEST_FUNCTIONS
+skip_if_skipped
 
 rm -f test-virt-format.img