Fix line and path separators in test

[som-snytt]

Fixes #23458

Splitting on NL leaves the CR if present. Just use linesIterator.

Introduce a template method for rendering paths. The test reporter replaces windows convention when run on windows, so that check files are correct; FileDiff.matches currently accounts for comparing lines of text when run on windows.

Previously, a TestReporter with empty console output would contribute a NL because "".split("\n") is not empty. (There is a test reporter per compilation group.)

[som-snytt]

A NL is dropped from check files of bootstrapped neg-macros but it's not obvious how to supply the update flag.

Edit: just scala3-bootstrapped/testCompilation --update-checkfiles and not testOnly.

[som-snytt]

Spurious windows_fast failure.

[som-snytt]

That's a nice idea! Did you actually try it again under windows to see if it akshually works?

Edit: the TestReporter's ConsoleReporter (its consoleOutput) is used for the check file, so that is where the path fix is required.

Probably the TestReporter is not used to report a path in a diagnostic, but it also translates paths, because why not.

The test rig reports "local" paths as usual, but the diagnostics don't include a path (which would be normalized).

Note, of course, how sbt "progress" collides with test rig output, which is still quite annoying and provides no value.

Windows example where a spurious // warn is reported:

Wrong number of warnings encountered when compiling tests\warn\i15503g.scala
expected: 8, actual: 7 / Test / testOnly 9s
Unfulfilled expectations:
tests\warn\i15503g.scala:27

-> following the diagnostics:
 at 8: unused explicit parameter
 at 9: unused implicit parameter
 at 10: unused explicit parameter
 at 10: unused implicit parameter
 at 11: unused explicit parameter
 at 23: unused explicit parameter in extension method isAnIssue
 at 29: unused implicit parameter in extension method repeat

In fact getMissingExpectedWarnings constructs the file path with File.separator.

Edit: that is, local convention is good for test rig reports; normalized path is required only for check file output.

[mbovel]

This PR fixes #23458:

1. 976a8f5: replaces String.split("\n") with linesIterator to properly handle Windows \r\n line endings, preventing \r characters from being left behind
   and causing formatting issues.
2. 5672e36: normalizes paths in test outputs using a new renderPath function to ensure consistent cross-platform test results.
3. 45afde7: removes extra leading newlines from 43 checkfiles.

Looks good to me, thanks!

[som-snytt]

This was not refreshed and is stale with respect to 2 months of new check files.

[mbovel]

This was not refreshed and is stale with respect to 2 months of new check files.

Do we except a lot of new check files to be affected?

For now the CI (test_non_bootstrapped and test_windows_full) seems to (also) fail for problems that were already occurring before the merge (see runs on the previous commit).

[som-snytt]

In the context of the current dust due to construction, if a few check files need --update-checkfiles, it will not be terrible.

I was feeling paranoid about breaking the build!