Test chars safely when highlighting

[som-snytt]

The arrow test at lastOffset - 3 is out-of-range when highlighting, which parses a snippet of source.

Fixes #22906

Reproduces under scala-cli -rewrite -indent but not under vulpix. (It would be nice to know why.) (Needless to add, it shouldn't be a research project to get the test rig to just run a compilation. There is programmatic flag-setting that defies expectations.)

This commit also allows a test to using scala by ignoring it, as a convenience. When the directive parsing is completely aligned with scala-cli, it should completely ignore what is not supported, so that test files can be used both ways.

[som-snytt]

It would be nice if the nuisance message did not make its way into the check file. The test rig could de-duplicate. Again, the goal is that I don't have to change my scala-cli reproduction just to use it as a test.

Output from 'tests/neg/i22906.scala' did not match check file. Actual output:
Flag -indent set repeatedly
-- Error: tests/neg/i22906.scala:5:15 ----------------------------------------------------------------------------------

[som-snytt]

This is nominally a no-op without -source 3.0-migration but would throw; it requires the other options to demonstrate the behavior.

[som-snytt]

updated to document my comment that vulpix doesn't demonstrate the error.

[som-snytt]

Squashed. Some tests such as skipBlanks could be written as indexWhere or lastIndexWhere (going backward). (Not done here.)

[som-snytt]

Fixed the line number in the check file.