emacs config: Make C-c C-c execute the current test through ./run

* .dir-locals.el: Redefine C-c C-c in sh-mode and python-mode for
the tests/ subdirectory.c
* HACKING: Mention it.

HACKING

@@ -109,6 +109,7 @@ another "magic touch":
 
   touch doc/org-stamp
 
+
 Silent Building with automake
 -----------------------------
 
@@ -120,6 +121,30 @@ annoying. To reduce this verbosity, just run:
 
   make V=0
 
+
+Executing a single test
+-----------------------
+
+All tests in subdirectories of tests/ are executed through the
+tests/run script.  That run script defines PATH and other environment
+variables necessary so that Shell and Python scripts will use the
+version of Spot under development.  For instance to execute
+tests/core/acc.test, do:
+
+   % cd tests
+   % ./run core/acc.test
+
+A test run this way automatically executes in verbose mode.  Any
+temporary file generated by the script can be found in
+tests/core/acc.dir/.
+
+Alternatively, if you use emacs, the .dir-locals.el configuration at
+the top of the project redefines the C-c C-c key for all shell and
+python scripts under the tests/ directory.  So if you are working on
+tests/core/acc.test under emacs, hitting C-c C-c should be all you
+need to execute it.
+
+
 Debugging Libtool executables
 -----------------------------