Reduce test suite runtime and run the Unicode tests everywhere

Squashed onto develop from:
- Cut Unicode ill-formed byte sweeps to one representative prefix
- Pin unrelated bytes in the remaining ill-formed UTF-8 sweeps
- Speed up unit-unicode1
- Run the cheap binary format size tests unconditionally
- Compile unit-msgpack.cpp only once
- Check the JSON Pointer roundtrip for every code point again
- Cover every byte class in the ill-formed UTF-8 sweeps
- Merge the Unicode tests into unit-unicode.cpp
- Stop excluding the Unicode tests in CI

Co-authored-by: elix3r <157088510+22elix3r@users.noreply.github.com>
Signed-off-by: elix3r <157088510+22elix3r@users.noreply.github.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/external_ci/appveyor.yml

@@ -87,9 +87,4 @@ build_script:
   - cmake --build . --config "%configuration%" --parallel 2
 
 test_script:
-  - if "%configuration%"=="Release" ctest -C "%configuration%" --parallel 2 --output-on-failure
-  # On Debug builds, skip test-unicode_all
-  # as it is extremely slow to run and cause
-  # occasional timeouts on AppVeyor.
-  # More info: https://github.com/nlohmann/json/pull/1570
-  - if "%configuration%"=="Debug" ctest --exclude-regex "test-unicode" -C "%configuration%" --parallel 2 --output-on-failure
+  - ctest -C "%configuration%" --parallel 2 --output-on-failure