[llvm] [LIT][Cygwin] Mark `ulimit -n` unsupported for Cygwin (PR #164604)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `llvm` at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/41678

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
11.743 [4/29/0] cd /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/runtimes/runtimes-bins && /home/buildbots/llvm-external-buildbots/cmake-3.31.2/bin/cmake --build /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/runtimes/runtimes-bins/ --target libomp-mod --config Release
ninja: no work to do.
12.071 [2/4/28] Building CXX object tools/flang/unittests/Frontend/CMakeFiles/FlangFrontendTests.dir/FrontendActionTest.cpp.o
12.533 [1/3/30] Linking CXX executable tools/flang/unittests/Common/FlangCommonTests
17.538 [1/2/31] Linking CXX executable tools/flang/unittests/Optimizer/FlangOptimizerTests
25.931 [1/1/32] Linking CXX executable tools/flang/unittests/Frontend/FlangFrontendTests
25.932 [0/1/32] Running the Flang regression tests
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1226.732348

```

</details>

https://github.com/llvm/llvm-project/pull/164604