Add documentation on how to create C++-level unit tests

testsuite/cpptests/README.md

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+# C++ unit tests
+
+This directory contains C++-level unit tests. The tests are run using the
+Boost.Test framework, and are compiled into a separate executable,
+`run_all_cpptests` by CMake if NEST is configured with Boost.
+
+Tests are written in header files, preferably one file per test suite. Each
+header file must include the Boost.Test header, create a test suite and at
+least one test. Validation of predicate values is done with the
+`BOOST_REQUIRE()` macro.
+
+A small example of a test suite header can look like this:
+
+```cpp
+#define BOOST_TEST_DYN_LINK
+#include <boost/test/unit_test.hpp>
+
+BOOST_AUTO_TEST_SUITE( foo_suite )
+
+BOOST_AUTO_TEST_CASE( foo_case )
+{
+  int i = 1;
+
+  BOOST_REQUIRE( i > 0 );  // passes
+  BOOST_REQUIRE( i == 2 ); // fails
+}
+
+BOOST_AUTO_TEST_SUITE_END()
+```
+
+The header then has to be included in `run_all.cpp` to be included in the
+executable.
+
+For further documentation on using the Boost.Test framework, see the [Boost.Test
+documentation](https://www.boost.org/doc/libs/1_69_0/libs/test/doc/html/index.html).