editorconfig: don't trim trailing whitespace in tests

[lolbinarycat]

some test snapshot files require trailing whitespace, and previously manually editing those snapshot files (as is required for run-make tests and some platform-specific tests) in an editor with editorconfig support would cause that whitespace to be removed, causing CI failures like this one

[rustbot]

r? @ehuss

rustbot has assigned @ehuss.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

Some changes occurred in src/tools/cargo

cc @ehuss

[mati865]

How about disabling it only for stdout and stderr files in tests?

[lolbinarycat]

@mati865 there's a few run-make tests that use other file extensions, so collecting a comprehensive list might be a bit tricky.

i think more reliable maybe would be disabling it for all tests/ files except those written in rust and js?

[mati865]

@lolbinarycat that sounds sensible to me but I'd leave the decision to the reviewer.