Add a test for object extraction from subprojects.

Commit ff51f2196f broke object extraction from subprojects. Add a new test case
to ensure this doesn't happen again.

Signed-off-by: Hemmo Nieminen <hemmo.nieminen@iki.fi>

test cases/common/84 extract subproject object/main.c

@@ -0,0 +1,5 @@
+int sub_lib_method(void);
+
+int main() {
+    return 1337 - sub_lib_method();
+}

test cases/common/84 extract subproject object/meson.build

@@ -0,0 +1,9 @@
+project('extract subproject object', 'c')
+
+sub = subproject('sub_project')
+lib = sub.get_variable('lib')
+
+exe = executable('exe', 'main.c',
+  objects : lib.extract_objects('sub_lib.c'))
+
+test('extraction test', exe)

test cases/common/84 extract subproject object/subprojects/sub_project/meson.build

@@ -0,0 +1,3 @@
+project('extract subproject object -- subproject', 'c')
+
+lib = shared_library('sub_lib', 'sub_lib.c')

test cases/common/84 extract subproject object/subprojects/sub_project/sub_lib.c

@@ -0,0 +1,3 @@
+int sub_lib_method() {
+    return 1337;
+}