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/configure.ac b/configure.ac
index 9a6f963..a1dbce5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -170,7 +170,10 @@
 				[Include the VTY/CTRL tests in make check [default=no]]),
 		[enable_ext_tests="$enableval"],[enable_ext_tests="no"])
 if test "x$enable_ext_tests" = "xyes" ; then
-	AM_PATH_PYTHON
+	AC_CHECK_PROG(PYTHON2_AVAIL,python2,yes)
+	 if test "x$PYTHON2_AVAIL" != "xyes" ; then
+		AC_MSG_ERROR([Please install python2 to run the VTY/CTRL tests.])
+	fi
 	AC_CHECK_PROG(OSMOTESTEXT_CHECK,osmotestvty.py,yes)
 	 if test "x$OSMOTESTEXT_CHECK" != "xyes" ; then
 		AC_MSG_ERROR([Please install git://osmocom.org/python/osmo-python-tests to run the VTY/CTRL tests.])