Add parseUInt utility function

There is an increasing number of places where Catch2 wants to parse
strings into numbers, but being stuck in C++14 world, we do not
have good stdlib facilities to do this (`strtoul` and `stoul`
are both bad).

tests/CMakeLists.txt

@@ -85,6 +85,7 @@ set(TEST_SOURCES
         ${SELF_TEST_DIR}/IntrospectiveTests/FloatingPoint.tests.cpp
         ${SELF_TEST_DIR}/IntrospectiveTests/GeneratorsImpl.tests.cpp
         ${SELF_TEST_DIR}/IntrospectiveTests/InternalBenchmark.tests.cpp
+        ${SELF_TEST_DIR}/IntrospectiveTests/Parse.tests.cpp
         ${SELF_TEST_DIR}/IntrospectiveTests/PartTracker.tests.cpp
         ${SELF_TEST_DIR}/IntrospectiveTests/RandomNumberGeneration.tests.cpp
         ${SELF_TEST_DIR}/IntrospectiveTests/Reporters.tests.cpp