restore disabled FileSystemTests (#199)
restore disabled FileSystemTests (#199)

* restore disabled FileSystemTests

motivation: better test coverage

changes:
* refactor the common part of testInMemoryFileSystemFileLock, testLocalFileSystemFileLock, testRerootedFileSystemViewFileLock
* resove the potential reader race by making sure file have content prior to starting the threads
* enable the above disabled tests in FileSystemTests

rdar://71560894

apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core