Document unit tests

Signed-off-by: Arne Schwabe <arne@openvpn.net>

test/unittests/README.md

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+# Unit test framework for OpenVPN3 #
+
+The unit test framework is written in the [Google Test](https://github.com/google/googletest)
+framework.
+
+## Building/running the unit tests ##
+Before building the unit tests themselves, you should build the dependencies
+as described in the [README.rst](../../README.rst).
+
+The unit test cmake files assume here that the deps directory is on the same
+level as the openvpn3 directory unless overridden by the DEP_DIR variable.
+
+Building unit tests (assuming you are in the openvpn3 directory):
+
+    ➜ mkdir ../unittests
+    ➜ cd ../unittests
+    ➜ cmake ../openvpn3
+    ➜ cmake --build . --target coreUnitTests
+
+Note: On Linux and Mac OS you can use `make coreUnitTests` instead of `cmake --build`
+
+Run the unit tests:
+
+    ➜  ./test/unittests/coreUnitTests --gtest_shuffle
+
+On a Mac with OpenSSL from [homebrew](brew.sh):
+
+    ➜ cmake ../openvpn3 -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl@1.1
+
+Using mbed TLS instead of OpenSSL
+
+    ➜ cmake ../openvpn3 -DUSE_MBEDTLS
+
+A full list of build options can shown with
+
+    ➜ cmake -LAH .
+
+Example for building and running on Windows:
+
+    ➜ cmake -DDEP_DIR=C:\o3\deps -DUSE_MBEDTLS=true -DCMAKE_GENERATOR_PLATFORM=x64 C:\o3\openvpn3
+    ➜ cmake --build . --target coreUnitTests
+    ➜ test\unittests\Debug\coreUnitTests.exe --gtest_output="xml:test_core.xml" --gtest_shuffle
+
+## Writing unit tetss ##
+
+Each new test suite should be a new a file called `test_suitename.cpp` and added to the
+`CMAKELists.txt` file. Eah test includes an `#include test_common.h` at the top to setup
+common openvpn3 library parameter and to set the `OPENVPN_EXTERN` define to `extern`. This
+is done so header files that define globals can be included multiple times. The only file that
+includes headers without the `OPENVPN_EXTERN` being set is the `core_tests.cpp` file. All
+global includes should be done in this file.
+
+Currently all tests can fit in the same compilation unit `coreUnitTests`. If a unit test
+requires special compile/includes or other options that are not compatible with the rest of
+the unit tests, another  compilation unit should be added to the CMAKELists.txt
+
+The `test_helper.cc` file adds helper functions that can be used for unit tests. See the file
+for more information.