Allow run-tests.sh to skip tests

This change allows tests to be skipped for development purposes by
setting the environment variable `ERT_SELECTOR' to a valid ERT test
selector, e.g.:

  ERT_SELECTOR='(not "-fixfn")' ./run-tests.sh

Refer to the docstrings for `ert-run-tests-batch-and-exit' and
`ert-select-tests' for details on how to compose test selectors.

run-tests.sh

@@ -6,13 +6,23 @@ if [ -z "$EMACS" ] ; then
     EMACS="emacs"
 fi
 
+# Run all tests by default.
+# To only run certain tests, set $ERT_SELECTOR as required.
+# For example, to skip the test "-fixfn", run the following command:
+#
+# ERT_SELECTOR='(not "-fixfn")' ./run-tests.sh
+#
+if [ -z "$ERT_SELECTOR" ] ; then
+    ERT_SELECTOR="nil"
+fi
+
 $EMACS -batch \
        $([[ $EMACS == "emacs23" ]] && echo -l dev/ert.el) \
        -l dash.el \
        -l dash-functional.el \
        -l dev/examples-to-tests.el \
        -l dev/examples.el \
-       -f ert-run-tests-batch-and-exit
+       --eval "(ert-run-tests-batch-and-exit (quote ${ERT_SELECTOR}))"
 
 if [[ $EMACS != "emacs23" ]]; then
     $EMACS -Q --batch \