runtests: allow skipping tests on torture, use for test 357

Some tests may take a long time in torture mode. Make it possible
to skip individual tests when runtests in running in torture mode.

Also:
- skip test 357 for the reason above.
  Saved 1-3 minutes for the Linux CI torture job, 1-1.5m on Windows.
  No savings on macOS.

Reported-by: Stefan Eissing
Fixes #21873

Closes #21906

docs/tests/FILEFORMAT.md

@@ -539,6 +539,7 @@ Features testable here are:
 - `SSPI`
 - `threaded-resolver`
 - `TLS-SRP`
+- `torture` - if runtests is running in memory test mode
 - `TrackMemory`
 - `typecheck`
 - `threadsafe`

docs/tests/TEST-SUITE.md

@@ -188,6 +188,9 @@ that memory leaks do not occur even in those situations. It can help to
 compile curl with `CPPFLAGS=-DMEMDEBUG_LOG_SYNC` when using this option, to
 ensure that the memory log file is properly written even if curl crashes.
 
+If a specific test takes a long time to run in memory test mode, you can
+disable it individually by adding `!torture` to its `<features>` section.
+
 ### Debug
 
 If a test case fails, you can conveniently get the script to invoke the