FileUtility abstraction

[kshyju]

We need to introduce an abstraction to enhance the FileUtility class usage.

The class exposes an Instance property

azure-functions-host/src/WebJobs.Script/Extensions/FileUtility.cs

Lines 21 to 24 in 0ee99cb

	public static IFileSystem Instance
	{
	get { return _instance ?? _default; }
	set { _instance = value; }

which I see many tests are overriding when running the tests with a mocked IFileSystem. But since FileUtility.Instance is a shared state, if multiple tests are running parallelly, this would cause one test to get the state set by another test.

azure-functions-host/test/WebJobs.Script.Tests/Security/SimpleKubernetesClientTests.cs

Line 64 in 0ee99cb

FileUtility.Instance = fileSystem.Object;

[jviau]

All these usages of FileUtility should be replaced with DI'ing IFileSystem.