Test filesystem api

* Invariant tests for the File interface
* Invariant tests for reading / writing files
** Due to missing features currently ignoring CryptoFileSystem

main/filesystem-inmemory/src/main/java/org/cryptomator/filesystem/inmem/InMemoryNode.java

@@ -46,6 +46,9 @@ class InMemoryNode implements Node {
 
 	@Override
 	public Instant lastModified() {
+		if (!exists()) {
+			throw new UncheckedIOException(new IOException("File does not exist"));
+		}
 		return lastModified;
 	}