Fix single object extraction test

The library from which we extract should have multiple sources
from which we choose a strict subset otherwise we're testing almost the
same case extracting all the objects.

This exposes a bug in how we were doing extract checking for unity
builds.

test cases/common/25 object extraction/lib2.c

@@ -0,0 +1,3 @@
+int retval() {
+  return 43;
+}

test cases/common/25 object extraction/meson.build

@@ -4,7 +4,7 @@ if meson.is_unity()
   message('Skipping extraction test because this is a Unity build.')
 else
   lib1 = shared_library('somelib', 'src/lib.c')
-  lib2 = shared_library('somelib2', 'lib.c')
+  lib2 = shared_library('somelib2', 'lib.c', 'lib2.c')
 
   obj1 = lib1.extract_objects('src/lib.c')
   obj2 = lib2.extract_objects(['lib.c'])