It fixes test 2302, 2303, 2307 with MSVC and clang on Windows.
GCC Windows builds were not affected.

Failure was caused by stack overflow due to a 1MB+ sized test struct on
stack. Replace it with dynamic allocation.

Also unignore affected tests in GHA/windows.

As seen under WINE with llvm-mingw:

$ wine64 libtests.exe lib2302 ws://127.0.0.1:59964/2302 > stdout2302 2> stderr2302
Test: lib2302
URL: ws://127.0.0.1:59964/2302
wine: Unhandled stack overflow at address 000000014007486A (thread 0024), starting debugger...
Unhandled exception: stack overflow in 64-bit code (0x000000014007486a).

Ref: #16629 (discovery)
Ref: 1bd5ac9 #16570

• rebase on GHA/windows: add GnuTLS job, drop default OpenSSL for Cygwin/MSYS2 jobs #16629