Start section in the programmer guide describing how to add unit tests

doc/src/Developer_unittest.rst

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+Adding tests for unit testing
+-----------------------------
+
+This section discusses adding or expanding tests for the unit test
+infrastructure included into the LAMMPS source code distribution.
+Unlike example inputs, unit tests focus on testing the "local" behavior
+of individual features, tend to run very fast, and should be set up to
+cover as much of the added code as possible.  When contributing code to
+the distribution, the LAMMPS developers will appreciate if additions
+to the integrated unit test facility are included.
+
+Given the complex nature of MD simulations (i.e. many operations can
+only be performed when suitable "real" simulation environment has been
+set up) not all tests will be unit tests in the strict definition of
+the term, but rather be executed on a more abstract level by issuing
+LAMMPS script commands and then inspecting the changes to the internal
+data.  For some classes of tests, generic test programs have been
+written that can be applied to parts of LAMMPS that use the same
+interface (via polymorphism) and those are driven by input files, so
+tests can be added by simply adding more of those input files.
+
+
+Adding tests for styles computing forces
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Adding tests for individual LAMMPS commands
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Adding tests for the C-style library interface
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Adding tests for the Python module
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Adding tests for the Fortran interface
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Adding tests for the C++-style library interface
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Adding tests for utility functions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Adding tests for programs in the tools folder
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^