We still want to build VS 2017 through AppVeyor, and those images
have CMake 3.16.2 installed. We could install newer CMake as part
of the build, but since we don't use newer CMake features yet, this
is simpler.

We also bump the minimum CMake version to 3.20 as per #2943

`gmtime*` on Windows fails on dates pre 1970, and because we didn't
check the return code, we would then pass invalid `tm` struct to
`strftime` causing it to assert.

Closes #2944

This was renamed to CATCH_REGISTER_ENUM in 541f1ed1 (Only provide
CATCH_REGISTER_ENUM, 2019-04-21).

Closes #2917

This allows writing something like

  const auto v = calculateSomeVectorOfInts();
  CHECK_THAT(v, RangeEquals({1, 2, 3}));

Fixes #2915.

The main reason for this is to be able to distinguish between
different errors (or "errors") based on the return code. Before
this change, it was impossible to use the exit code to figure out
whether a test binary failed because all tests were skipped or
because exactly 4 assertions have failed.

This meant that using `catch_discover_tests` and telling it to
check for exit code == 4 to determine skipped tests could lead to
false negatives.

When presented with just '-' it would access the string at position [1]

This mirrors the changes done to the JUnit reporter in commit
fe483c056d9516d3550519c29fbf14a4d84d06d4