Explain how to build the tests

It may not be obvious how to build the tests, so add a section
explaining this to the existing file containing the instructions about
writing the tests and add a README to the tests directory itself to
increase chances that this file will be found -- hopefully anybody
interested in the tests will look at the README in the directory
containing them.

docs/contributing/how-to-write-unit-tests.md

@@ -9,6 +9,33 @@ included in wxWidgets as a submodule, so you will need to run
 to get it before the first use. Catch is header-only and doesn't need to be
 compiled.
 
+Building the tests
+------------------
+
+Before starting modifying the tests, please make sure you can build the
+existing tests. This requires having built the library itself successfully
+first and the way to build the test must correspond to the way you used to
+build the library:
+
+- When using MSVS under MSW: just open the provided `tests/test_vcX.sln` (for
+  non-GUI tests) or `tests/test_gui_vcX.sln` (for the GUI ones) solution file
+  (where `X` corresponds to the version of MSVS you use, e.g. 16 for MSVS 2019)
+  and build it in the configuration of your choice.
+
+- When using configure under Unix or in a Unix-like environment, such as MSYS:
+  go to the `tests` subdirectory under the _build_ directory (i.e. the
+  directory where you ran configure, not the one in the source tree) and run
+  `make test` (non-GUI tests) or `make test_gui` (GUI ones) to build.
+
+- When using `makefile.vc` or `makefile.gcc` under MSW to build the libraries,
+  use the same makefile under `tests` to build the tests.
+
+
+Once the tests were built successfully, you can run them to check that
+everything works correctly by simply launching the corresponding test binary.
+See the last subsection for more details about running the tests.
+
+
 Testing with Catch
 ------------------
 

tests/README.md

@@ -0,0 +1,13 @@
+Tests
+-----
+
+This directory contains tests for the library and is mostly useful for the
+library developers. See the `samples` subdirectory for the examples that are
+more useful to the application developers using the library.
+
+If you do work on the library itself and would like to modify an existing or
+add a new test, please see `docs/contributing/how-to-write-unit-tests.md` for
+more information.
+
+This file also contains the instructions for running the tests if you'd just
+like to do it to confirm that the library works correctly.