patch 8.1.0105: all tab stops are the same

Problem:    All tab stops are the same.
Solution:   Add the variable tabstop feature. (Christian Brabandt,
            closes #2711)
diff --git a/src/Makefile b/src/Makefile
index ca97254..e9a15a0 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2305,6 +2305,7 @@
 	test_usercommands \
 	test_utf8 \
 	test_utf8_comparisons \
+	test_vartabs \
 	test_viminfo \
 	test_vimscript \
 	test_virtualedit \