php8*: disable PCRE2 JIT SEAlloc to avoid crashes when forking

This is a follow up to #200815 and #184634.

The PCRE2 JIT SEAlloc does not support the `fork()` as announced in
their README [0]:
> If you are enabling JIT under SELinux environment you may also want to add
>  --enable-jit-sealloc, which enables the use of an executable memory allocator
>  that is compatible with SELinux. Warning: this allocator is experimental!
>  It does not support fork() operation and may crash when no disk space is
>  available. This option has no effect if JIT is disabled.

As a result using it in PHP can break apps and tools, it can only be
enabled under very specific context where you have a full picture of
what the PHP code is doing.

This contribution disables again the PCRE2 JIT SEAlloc and extends the
existing PHP/PCRE2 tests to make sure we do not enable it again by
mistake.

[0] https://www.pcre.org/readme.txt

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