patch 8.1.0496: no tests for indent files

Problem:    No tests for indent files.
Solution:   Add a mechanism for running indent file tests.  Add a first test
            for Vim indenting.

runtime/indent/Makefile

@@ -0,0 +1,13 @@
+# Portable Makefile for running indent tests.
+
+VIM = vim
+
+# Run the tests that didn't run yet or failed previously.
+# If a test succeeds a testdir/*.out file will be written.
+# If a test fails a testdir/*.fail file will be written.
+test:
+	$(VIM) --not-a-term -u testdir/runtest.vim
+
+
+clean:
+	$(VIM) --not-a-term -u testdir/cleantest.vim