Test for spurious warning from get_target_filename

Fixed by #7494 without a test, so here's a test.

Extracted from https://github.com/mesonbuild/meson/pull/7539

run_unittests.py

@@ -5029,6 +5029,14 @@ recommended as it is not supported on some platforms''')
             self.build()
             self.run_tests()
 
+    def test_multi_output_custom_target_no_warning(self):
+        testdir = os.path.join(self.common_test_dir, '235 custom_target source')
+
+        out = self.init(testdir)
+        self.assertNotRegex(out, 'WARNING:.*Using the first one.')
+        self.build()
+        self.run_tests()
+
 class FailureTests(BasePlatformTests):
     '''
     Tests that test failure conditions. Build files here should be dynamically

test cases/common/235 custom_target source/meson.build

@@ -0,0 +1,5 @@
+project('a', ['c'])
+
+x = find_program('x.py')
+outs = custom_target('foo', output: ['x.c', 'y'], input: 'a', command: [x])
+executable('testprog', outs[0])

test cases/common/235 custom_target source/x.py

@@ -0,0 +1,5 @@
+#! /usr/bin/env python3
+with open('x.c', 'w') as f:
+    print('int main(void) { return 0; }', file=f)
+with open('y', 'w'):
+    pass