Rewrite BitReaderTest, NFC

Rewrite the single unit test in `BitReaderTest` so that it's easier to
add more tests.

  - Parse from an assembly string rather than using API.
  - Use more helper functions.
  - Use a separate context for the module on the other side.

Aside from relying on the assembly parser, there's no functionality
change intended.

llvm-svn: 214556