tests: build_by_default : false targets used in tests should be built

The next commit fixes this.

test cases/common/136 build by default targets in tests/main.c

@@ -0,0 +1,3 @@
+int main (int argc, char *argv[]) {
+  return 0;
+}

test cases/common/136 build by default targets in tests/meson.build

@@ -0,0 +1,23 @@
+project('unit-test', 'c', version : '1.0')
+
+write_file = find_program('write_file.py')
+
+# A test that consumes and verifies the output generated by a custom target.
+# Should work even if target is not built by default. Makes sure that foo.out
+# is actually created before the test command that uses foo_out is run.
+foo_out = custom_target('foo.out',
+  output : 'foo.out',
+  command : [write_file, '@OUTPUT@'])
+
+# Also verify that a build_by_default : false BuildTarget added to a test is
+# built before the test is run.
+exe_out = executable('out', 'main.c', build_by_default : false)
+
+py_file_exists = '''import os, sys
+if not os.path.exists(sys.argv[1]) or not os.path.exists(sys.argv[2]):
+  print("could not find {!r} or {!r} in {!r}"
+        "".format(sys.argv[1], sys.argv[2], os.getcwd()))
+  sys.exit(1)'''
+
+python = import('python3').find_python()
+test('output-check', python, args : ['-c', py_file_exists, foo_out, exe_out])

test cases/common/136 build by default targets in tests/write_file.py

@@ -0,0 +1,6 @@
+#!/usr/bin/env python3
+
+import sys
+
+with open(sys.argv[1], 'w') as f:
+    f.write('Test')