tests: Let run_tests.sh run Python tests without Makefile assistance

Currently run_tests.sh needs the $PYTHON environment variable set to
correctly run pylibfdt tests.  The Makefile does this for make check, but
it breaks if the script is run manually.  Add a fallback to handle that
case.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/run_tests.sh

@@ -7,6 +7,10 @@ if [ -z "$CC" ]; then
     CC=cc
 fi
 
+if [ -z "$PYTHON" ]; then
+    PYTHON=python3
+fi
+
 # stat differs between platforms
 if [ -z "$STATSZ" ]; then
 	stat --version 2>/dev/null | grep -q 'GNU'