Catch `catch_discover_tests`

Copied from [Dynamic Catch test discovery in CMake](https://gist.github.com/garethsb/a01ed0dbd4977d439c16200640549935), which was inspired by [Dynamic Google Test Discovery in CMake 3.10](https://blog.kitware.com/dynamic-google-test-discovery-in-cmake-3-10/).

Original source code:

- Adapted by [Gareth Sylvester-Bradley](https://github.com/garethsb) from [GoogleTest ``gtest_discover_tests``](https://gitlab.kitware.com/cmake/cmake/merge_requests/1056).
- BSD 3-clause "New" or "Revised" License.
- Copyright 2000-2017 Kitware, Inc. and Contributors. All rights reserved.

contrib/CatchAddTests.cmake

@@ -0,0 +1,77 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+set(prefix "${TEST_PREFIX}")
+set(suffix "${TEST_SUFFIX}")
+set(spec ${TEST_SPEC})
+set(extra_args ${TEST_EXTRA_ARGS})
+set(properties ${TEST_PROPERTIES})
+set(script)
+set(suite)
+set(tests)
+
+function(add_command NAME)
+  set(_args "")
+  foreach(_arg ${ARGN})
+    if(_arg MATCHES "[^-./:a-zA-Z0-9_]")
+      set(_args "${_args} [==[${_arg}]==]") # form a bracket_argument
+    else()
+      set(_args "${_args} ${_arg}")
+    endif()
+  endforeach()
+  set(script "${script}${NAME}(${_args})\n" PARENT_SCOPE)
+endfunction()
+
+# Run test executable to get list of available tests
+if(NOT EXISTS "${TEST_EXECUTABLE}")
+  message(FATAL_ERROR
+    "Specified test executable '${TEST_EXECUTABLE}' does not exist"
+  )
+endif()
+execute_process(
+  COMMAND ${TEST_EXECUTOR} "${TEST_EXECUTABLE}" ${spec} --list-test-names-only
+  OUTPUT_VARIABLE output
+  RESULT_VARIABLE result
+)
+# Catch --list-test-names-only reports the number of tests, so 0 is... surprising
+if(${result} EQUAL 0)
+  message(WARNING
+    "Test executable '${TEST_EXECUTABLE}' contains no tests!\n"
+  )
+elseif(${result} LESS 0)
+  message(FATAL_ERROR
+    "Error running test executable '${TEST_EXECUTABLE}':\n"
+    "  Result: ${result}\n"
+    "  Output: ${output}\n"
+  )
+endif()
+
+string(REPLACE "\n" ";" output "${output}")
+
+# Parse output
+foreach(line ${output})
+  # Test name; strip spaces to get just the name...
+  string(REGEX REPLACE " +" "" test "${line}")
+  # ...and add to script
+  add_command(add_test
+    "${prefix}${test}${suffix}"
+    ${TEST_EXECUTOR}
+    "${TEST_EXECUTABLE}"
+    "${test}"
+    ${extra_args}
+  )
+  add_command(set_tests_properties
+    "${prefix}${test}${suffix}"
+    PROPERTIES
+    WORKING_DIRECTORY "${TEST_WORKING_DIR}"
+    ${properties}
+  )
+  list(APPEND tests "${prefix}${test}${suffix}")
+endforeach()
+
+# Create a list of all discovered tests, which users may use to e.g. set
+# properties on the tests
+add_command(set ${TEST_LIST} ${tests})
+
+# Write CTest script
+file(WRITE "${CTEST_FILE}" "${script}")