patch 8.1.2087: cannot easily select one test function to execute

Problem:    Cannot easily select one test function to execute.
Solution:   Support the $TEST_FILTER environment variable. (Ozaki Kiichi,
            closes #2695)
diff --git a/src/Makefile b/src/Makefile
index 75085ee..6e676f1 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2262,6 +2262,10 @@
 
 # Run individual NEW style test.
 # These do not depend on the executable, compile it when needed.
+# Set $TEST_FILTER to select what test function to invoke, e.g.:
+#	export TEST_FILTER=Test_terminal_wipe_buffer
+# A partial match also works:
+#	export TEST_FILTER=wipe_buffer
 $(NEW_TESTS):
 	cd testdir; $(MAKE) $@ VIMPROG=../$(VIMTESTTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)