Go: Add test for FP in go/unhandled-writable-file-close

[owen-mc]

Converts the tests to use be inline expectation tests and adds a test (marked SPURIOUS) for an FP when there is a deferred call to os.File.Close and then later a call to os.File.Sync.

What we should probably do in this case is to make sure that every control flow path from the deferred call to os.File.Close to the end of the function either includes a call to os.File.Sync or involves returning a non-nil error. (I thought about how to do this and I think you would define some predicates like this:

private BasicBlock allowedSuccessor(BasicBlock b) {
  result = b.getASuccessor()
  // and result doesn't contain a handled sync call
  // and result doesn't end by returning an non-nil error
}

private BasicBlock maximalAllowedSuccessor(BasicBlock b) {
  result = maximalAllowedSuccessor(allowedSuccessor(b))
  or
  not exists(allowedSuccessor(b)) and result = b
}

Then isCloseSink should require that when closeCall is deferred, maximalAllowedSuccessor(closeCall) does not include the exit block of the function