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.