[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