support xapian ngrams

Xapian supports an "ngrams" option to help with languages/scripts
without explicit wordbreaks, such as Chinese / Japanese / Korean.

Add some plumbing for supporting this in mu as well. Experimental for
now.

lib/utils/mu-test-utils.cc

@@ -94,9 +94,12 @@ Mu::mu_test_init(int *argc, char ***argv)
 {
 	const auto tmpdir{test_random_tmpdir()};
 
+	g_unsetenv("XAPIAN_CJK_NGRAM");
 	g_setenv("MU_TEST", "yes", TRUE);
 	g_setenv("XDG_CACHE_HOME", tmpdir.c_str(), TRUE);
 
+	setlocale(LC_ALL, "");
+
 	g_test_init(argc, argv, NULL);
 
 	g_test_bug_base("https://github.com/djcb/mu/issues/");