tests: Fix selection of python version

According to documentation (and personal experience), AM_PATH_PYTHON
selects the highest version of python, no matter if major version is
different, which means if both python2 and 3 are available, 3 will be
chosen an PYTHON will point to "/.../python" which is python3. Apparently,
the macro cannot be easily used to pick highest python2 version.

As {vty,ctrl}_test_runner.py require python2 and are incompatible with
python3, let's instead rely on the system having a "python2" binary
available, which is the case in most distros.

Change-Id: If8e57924ed2c8da7ab7692f58a4bb5c5a970484f
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 10bb916..3cd8bff 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -56,8 +56,8 @@
 python-tests: $(BUILT_SOURCES)
 	osmotestvty.py -p $(abs_top_srcdir) -w $(abs_top_builddir) -v
 	osmotestconfig.py -p $(abs_top_srcdir) -w $(abs_top_builddir) -v
-	$(PYTHON) $(srcdir)/vty_test_runner.py -w $(abs_top_builddir) -v
-	$(PYTHON) $(srcdir)/ctrl_test_runner.py -w $(abs_top_builddir) -v
+	$(srcdir)/vty_test_runner.py -w $(abs_top_builddir) -v
+	$(srcdir)/ctrl_test_runner.py -w $(abs_top_builddir) -v
 	rm -f $(top_builddir)/sms.db $(top_builddir)/gsn_restart $(top_builddir)/gtphub_restart_count
 else
 python-tests: $(BUILT_SOURCES)