Increase count for STM Weak test parallel

[jmid]

As #406 shows, the parallel STM Weak test occasionally fails to trigger on s390x, thus resulting in a false alarm.

This 1-character PR therefore bumps the test count.
Since this is a negative test stopping on the first counterexample, all platforms except s390x should be unaffected.
On Linux/s390x this should however increase the chance of triggering the issue.

[jmid]

CI summary:

• Bytecode trunk aborted domain_spawntree with Fatal error: Failed to create domain [ocaml5-issue] Fatal error: Failed to create domain on s390x, MinGW, Cygwin, ... #428
• Cygwin 5.1 timed out in Lin Dynlink test [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• Cygwin 5.1 timed out in Lin Dynlink test [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• Cygwin trunk aborted domain_spawntree with Fatal error: Failed to create domain [ocaml5-issue] Fatal error: Failed to create domain on s390x, MinGW, Cygwin, ... #428
• Cygwin trunk aborted domain_spawntree with Fatal error: Failed to create domain [ocaml5-issue] Fatal error: Failed to create domain on s390x, MinGW, Cygwin, ... #428
• MinGW bytecode trunk aborted domain_spawntree with Fatal error: Failed to create domain [ocaml5-issue] Fatal error: Failed to create domain on s390x, MinGW, Cygwin, ... #428
• MinGW trunk mysteriously cancelled underway

Out of 59 workflows 7 failed with 6 genuine failures and 1 CI issue.

Note:

• no false alarms (yay! 🎉)
• no errors related to the adjusted parallel STM Weak test

[jmid]

CI summary for merge to main

• Cygwin trunk aborted in domain_spawntree with Fatal error: Failed to create domain [ocaml5-issue] Fatal error: Failed to create domain on s390x, MinGW, Cygwin, ... #428
• Linux trunk aborted in domain_spawntree with Fatal error: Failed to create domain [ocaml5-issue] Fatal error: Failed to create domain on s390x, MinGW, Cygwin, ... #428

Out of 36 workflows 2 failed - both with genuine issues (no false alarms! 🎉)