[Support][test] Fix OpenDirectoryAsFileForRead test on AIX and z/OS (llvm#216241)

Commit 9c7ba7b ("[AIX][SystemZ][Support] Check if file is dir on
open instead of read") moved the `fstat`/`EISDIR` check from
`readNativeFile()` to `openNativeFileForRead()` on AIX and z/OS. This
means `openNativeFileForRead()` now returns `EISDIR` immediately on
those
platforms, but the test `FileSystemTest.OpenDirectoryAsFileForRead` was
not updated to match, causing it to fail at the
`ASSERT_THAT_EXPECTED(FD, Succeeded())` assertion.

Add a `#elif defined(_AIX) || defined(__MVS__)` branch to the test that
expects the error to be returned from `openNativeFileForRead()` rather
than from `readNativeFile()`, consistent with the behavior introduced by
that commit.

Author: DanielCChen

llvm/unittests/Support/Path.cpp

@@ -2040,7 +2040,7 @@ TEST_F(FileSystemTest, SetLastAccessAndModificationTimeDirectory) {
 TEST_F(FileSystemTest, OpenDirectoryAsFileForRead) {
   std::string Buf(5, '?');
   Expected<fs::file_t> FD = fs::openNativeFileForRead(TestDirectory);
-#ifdef _WIN32
+#if defined(_WIN32) || defined(_AIX) || defined(__MVS__)
   EXPECT_EQ(errorToErrorCode(FD.takeError()), errc::is_a_directory);
 #else
   ASSERT_THAT_EXPECTED(FD, Succeeded());