Chore: renaming Lin Internal tests

[jmid]

This PR cleans up a bit of historical bagage:
Lin.Internal was once known as Lin and lin_tests.ml-files named accordingly.
We thus rename these to lin_internal_tests.ml and adjust their test titles accordingly.
The latter should help separate one from the other in the CI logs (I for one, have confused myself over this).

Overall. the PR should however not change the functionality of the test suite.

[jmid]

CI summary for cb27759

• 32bit 5.1 - Lin.Internal CList int test with Thread found an unexpected counterexample Lin.Internal Thread int CList test finds unexpected counterexample under bytecode #358
• Cygwin trunk part1 - timeout during Lin Dynlink [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• Mingw bytecode trunk - timeout during threadomain [ocaml5-issue] Windows failures on threadomain #203
• linux-s390x-5.2 failed to install qcheck-core (fixed upstream)
• linux-ppc64le-5.2 failed to install qcheck-core (fixed upstream)
• 7 multicoretests-ci jobs cancelled underway because a new commit was pushed

Of the 27 completed runs 5 failed: 2 failures were genuine, 1 was a false alarm, and 2 are caching related

[jmid]

CI summary for c332d03

• Cygwin trunk/5.2 part 1 - timeout during negative Lin DSL Dynlink test with Domain [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• Mingw bytecode 5.1 - Lin.Internal CList int test with Thread found an unexpected counterexample Lin.Internal Thread int CList test finds unexpected counterexample under bytecode #358
• linux-arm64-5.2 failed to install qcheck-core (fixed upstream)
• linux-s390x-5.2 failed to install qcheck-core (fixed upstream)
• linux-ppc64le-5.2 failed to install qcheck-core (fixed upstream)

Of the 56 completed runs 5 failed: 1 failures was genuine, 1 was a false alarm, and 3 due to CI-caching.
I'll merge.

[jmid]

CI summary for the merge into main:

• the opam install workflows failed because 5.1.0+trunk is set to not available with the release of 5.1.0 (fixed in Bump 510 workflows to the 5.1.0 release #398)
• Mingw bytecode trunk timed out during threadomain [ocaml5-issue] Windows failures on threadomain #203
• Cygwin trunk part 1 timed out during negative Lin DSL Dynlink test with Domain [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307
• Cygwin trunk part 2 timeout during a 6701.6s Lin.Internal CList int64 test with Thread Timeouts during 'Lin.Internal CList int64 test with Thread' under Cygwin #397
• linux-ppc64le-5.2 failed with sequential STM Array, Bytes, and Float.Array aborts and crashes [ocaml5-issue] Crashes and hangs on ppc64 trunk/5.2 #380

Of the 37 completed runs 9 failed: 5 were opam failures, 3 failures was genuine, 1 is in the gray zone.