[lit] Throw in unimplemented method (NFC)

Summary:
lit's `OneCommandFileTest` class implements an abstract method that
raises if called. However, it raises by referencing an undefined
symbol. Instead, raise explicitly by throwing a `NotImplementedError`.
This is clearer, and appeases Python linters.

Patch By Brian Gesiak!

Reviewers: ddunbar, echristo, beanz

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D25170

llvm-svn: 283090

llvm/utils/lit/lit/formats/base.py

@@ -80,7 +80,7 @@ class OneCommandPerFileTest(TestFormat):
                 yield test
 
     def createTempInput(self, tmp, test):
-        abstract
+        raise NotImplementedError('This is an abstract method.')
 
     def execute(self, test, litConfig):
         if test.config.unsupported: