This is just some minor code cleanup for the parse_and_replace test,
there should not be any functional differences.

This adds an environment variable to make it easier to add new tests.

If rustfix received a suggestion which inserts new lines without
replacing existing lines, it would ignore the suggestion. This is
because `parse_snippet` would immediately return if the `lines` to
replace was empty.

The solution here is to just drop the code which messes with the
original text line. `cargo fix` (and compiletest) currently do not use
this. This was originally added back in the days when rustfix supported
an interactive UI which showed color highlighting of what it looks like
with the replacement. My feeling is that when we add something like this
back in, I would prefer to instead use a real diff library and display
instead of trying to do various text manipulation for display. This
particular code has generally been buggy, and has been a problem several
times.

The included test fails without this fix because the changes do not
apply, and the code cannot compile.

Suggestions that come from rustc that are multi-line only use LF line
endings. But if the file is checked out on windows with CRLF
line-endings, then you end up with a mix of line endings that don't
match the "fixed.rs" file.

Tracking this at rust-lang/rust#119482.