CI: improvements in test reliability and performance

- CI default: raise parallelism to 20
- CI valgind: set parallelism to 6
- CI non-native: adapt parallelism for OS builds
- CI Windows: no longer ignore FTP, TFTP, MQTT and SMTP
- CI Windows: restrict test timeout to 10 minutes
- CI Windows: do not run tests for msh3 build
- tests, various: restrict curl invocation to ipv4 to avoid talking to
  any ipv6 test server running in parallel
- tests: requiring http/2 server no longer needs to mention http server
- test 190: use a fixed timeout of 10 seconds instead of %FTPTIME2, as
  that value rises under parallel load beyond what the server is waiting
  and then produces different results
- test 1540: add debug logging
- testrunner: add verify checks for http/2 and http/3 server

Closes #15040

.github/workflows/linux.yml

@@ -649,7 +649,7 @@ jobs:
         run: |
           export TFLAGS='${{ matrix.build.tflags }}'
           if [[ '${{ matrix.build.install_packages }}' = *'valgrind'* ]]; then
-            TFLAGS+=' -j4'
+            TFLAGS+=' -j6'
           fi
           if [[ '${{ matrix.build.install_packages }}' = *'heimdal-dev'* ]]; then
             TFLAGS+=' ~2077 ~2078'  # valgrind errors