patch 8.1.0817: ":=" command is not tested

Problem:    ":=" command is not tested.
Solution:   Add a test. (Dominique Pelle, closes #3859)
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 39b6889..5857a22 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -105,6 +105,7 @@
 	test_erasebackword \
 	test_escaped_glob \
 	test_eval_stuff \
+	test_ex_equal \
 	test_ex_undo \
 	test_ex_z \
 	test_exit \