Previously tests would not fail if they encountered an error such as
LineOverflow or TrailingWhitespace. Making the tests error out will fix
this mismatch between running rustfmt for real and running the tests.
This also modifies all tests that previously contained errors so that
they no longer contain errors (in almost all of the tests this is
accomplished by setting error_on_line_overflow = false).
This commit adds a very rough implementation of handling the specified
line ranges in `config.file_lines_map` for statements. It reformats a
statement if its span is fully contained in the set of lines specified
for the file.
The implementation here is intended as a proof of concept, and
demonstration that the machinery added in the preceding commits is
functional. A final implementation would likely hook in via the
`Rewrite` trait.
Refs #434
