Allow overriding python for running tests, as well choosing tests

The first arg to run-tests.sh is used as a regex to narrow down
tests to run.  We should extend this to consider all args as
regexes.

Eg:

  ./run-tests.sh xml

run-tests.sh

@@ -2,12 +2,15 @@
 cd Lib
 PYTHONPATH=".:$PYTHONPATH"
 export PYTHONPATH
-TESTS=`grep -r --include='*.py' -l -e doctest -e unittest *`
+test "x$PYTHON" = x && PYTHON=python
+FILTER=$1
+test "x$FILTER" = x && FILTER=.
+TESTS=`grep -r --include='*.py' -l -e doctest -e unittest * | grep "$FILTER"`
 ret=0
 for test in $TESTS; do
 	echo "Running tests in $test"
 	test=`echo "$test" | sed 's@[/\\]@.@g;s@[.]py$@@'`
-	if ! python -m $test -v; then
+	if ! $PYTHON -m $test -v; then
 		ret=$((ret+1))
 	fi
 done