Tweak coverage-html target

- Don't try to rm -rf the directory before we start: somebody might
   have set it to ~, which would be quite sad.

 - Always quote the directory name

 - Use 'make reset-gcov' before running tests.

 - Use 'make check', not ./src/test/test

changes/coverage-html

@@ -0,0 +1,5 @@
+  o Minor features (testing):
+
+    - Add a "coverage-html" make target to generate HTML-visualized
+      coverage results when building with --enable-coverage. (Requires lcov.)
+      Patch from Kevin Murray.