Add g_test_build_filename()

This function allows testcases to find data files in various situations
of srcdir == builddir, srcdir != builddir and for installed tests.

https://bugzilla.gnome.org/show_bug.cgi?id=549783

docs/reference/glib/glib-sections.txt

@@ -2916,6 +2916,9 @@ g_test_add_data_func
 g_test_add_data_func_full
 g_test_add
 
+GTestFileType
+g_test_build_filename
+
 g_test_fail
 g_test_message
 g_test_bug_base