Add section to documentation about running tests

There was already a small amount of information about running the
regression tests in the folder walkthrough, which I've moved to the new
Running Tests section. The folder walkthrough now links to that section.

Also added the `unit/` folder to the folder walkthrough.

Author: Owen Jones

doc/architectural/compilation-and-development.md

@@ -17,7 +17,29 @@ To be documented.
 
 To be documented.
 
-## Documentation
+## Running tests ##
+
+### Regression tests ###
+
+The regression tests are contained in the `regression/` folder.
+They are grouped into directories for each of the tools/modules.
+Each of these contains a directory per test case, containing a C or C++
+file that triggers the bug and a `.desc` file that describes
+the tests, expected output and so on. There is a Perl script,
+`test.pl` that is used to invoke the tests as:
+
+    ../test.pl -c PATH_TO_CBMC
+
+The `–help` option gives instructions for use and the
+format of the description files.
+
+To be documented further.
+
+### Unit tests ###
+
+To be documented.
+
+## Documentation ##
 
 Apart from the (user-orientated) CBMC user manual and this document, most
 of the rest of the documentation is inline in the code as `doxygen` and

doc/architectural/folder-walkthrough.md

@@ -75,14 +75,10 @@ into the `doc/html` directory when running `doxygen` from `src`.
 
 ## `regression/` ##
 
-The `regression/` directory contains the test suites.
-They are grouped into directories for each of the tools/modules.
-Each of these contains a directory per test case, containing a C or C++
-file that triggers the bug and a `.desc` file that describes
-the tests, expected output and so on. There is a Perl script,
-`test.pl` that is used to invoke the tests as:
+The `regression/` directory contains the regression test suites. How to run the
+regression tests is documented in \ref compilation-and-development
 
-    ../test.pl -c PATH_TO_CBMC
+## `unit/` ##
 
-The `–help` option gives instructions for use and the
-format of the description files.
+The `unit/` directory contains the unit test suites. How to run the unit
+tests is documented in \ref compilation-and-development.