tests: re-enable 1510, unignore 2027 2051 in GHA/macos, document heimdal memleak

- test1510: it was marked flaky, then disabled. Keep the flaky flag
  and allow it to run again.
  Ref: ef02da3156
  Ref: 0110cced2a

- GHA/linux: extend information about the heimdal valgrind failures.

- GHA/macos: unignore 2037 2041 in torture tests. They no longer
  appear flaky.
  Follow-up to 4b791dca37 #14667

Closes #17462

.github/workflows/linux.yml

@@ -680,7 +680,7 @@ jobs:
               TFLAGS+=' -j6'
             fi
             if [[ '${{ matrix.build.install_packages }}' = *'heimdal-dev'* ]]; then
-              TFLAGS+=' ~2077 ~2078'  # valgrind errors
+              TFLAGS+=' ~2077 ~2078'  # valgrind reporting memory leaks from Curl_auth_decode_spnego_message() -> gss_import_name()
             fi
           fi
           [ -x "$HOME/venv/bin/activate" ] && source $HOME/venv/bin/activate