externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c598a9a517d7bdec8ba1a0bace3ef10' as 'externals/catch'

externals/catch/tools/misc/appveyorTestRunScript.bat

@@ -0,0 +1,17 @@
+SETLOCAL EnableDelayedExpansion
+
+rem Disable launching the JIT debugger for ctest.exe
+reg add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug\AutoExclusionList" /v "ctest.ext" /t REG_DWORD /d 1
+cd Build
+if "%CONFIGURATION%"=="Debug" (
+  if "%coverage%"=="1" (
+    ctest -j 2 -C %CONFIGURATION% -D ExperimentalMemCheck || exit /b !ERRORLEVEL!
+    python ..\tools\misc\appveyorMergeCoverageScript.py || exit /b !ERRORLEVEL!
+    codecov --root .. --no-color --disable gcov -f cobertura.xml -t %CODECOV_TOKEN% || exit /b !ERRORLEVEL!
+  ) else (
+    ctest -j 2 -C %CONFIGURATION% || exit /b !ERRORLEVEL!
+  )
+)
+if "%CONFIGURATION%"=="Release" (
+  ctest -j 2 -C %CONFIGURATION% || exit /b !ERRORLEVEL!
+)