Add this thing I forgot.

test cases/common/77 external test program/meson.build

@@ -0,0 +1,3 @@
+project('test is external', 'c')
+
+test('external', find_program('mytest.py'), args : ['correct'])

test cases/common/77 external test program/mytest.py

@@ -0,0 +1,9 @@
+#!/usr/bin/env python3
+
+import sys
+
+if sys.argv[1] == 'correct':
+    print('Argument is correct.')
+    sys.exit(0)
+print('Argument is incorrect:', sys.argv[1])
+sys.exit(1)