Add basic xz tests.

test/completion/xz.exp

@@ -0,0 +1 @@
+assert_source_completions xz

test/lib/completions/xz.exp

@@ -0,0 +1,20 @@
+proc setup {} {
+    save_env
+}; # setup()
+
+
+proc teardown {} {
+    assert_env_unmodified
+}; # teardown()
+
+
+setup
+
+
+assert_complete_any "xz "
+
+
+sync_after_int
+
+
+teardown