HACKING: Clarify unit test section

The "Working on the non-web parts of Cockpit" section is mostly
concerned about running the unit tests, with a little bit of how to
build the source tree. Rename it, to disambiguate it from the later
"Working on your local machine" section.

HACKING.md

@@ -119,7 +119,7 @@ remove the symlink with the following command and log back into Cockpit:
 
     rm ~/.local/share/cockpit
 
-## Working on the non-web parts of Cockpit
+## Building and unit tests
 
 Cockpit uses autotools, so there are familiar `./configure` script and
 Makefile targets.