Skip test of legacy compiler if not JDK 17

[som-snytt]

Fixes #23090

How much have your relied on LLM-based tools in this contribution?

LLM-free.

That is reminiscent of Ollie, ollie, in come free, which is how we said it.

How was the solution tested?

Manually tested locally under JDK 25, to show that

tests/run/backwardsCompat-implicitParens/A_1_c3.0.2.scala

causes that test to be skipped.

Single test files were not compiled correctly (with the legacy compiler), so also test that this fails without the fix and is skipped with the fix:

tests/run/lazyVals_c3.1.0.scala

Additional notes

Left the unused import of chaining because lacking it results in a tangential rabbit hole:

#25451

[som-snytt]

It should also skip "individual" test files.

Edit: tests such as tests/run/lazyVals_c3.1.0.scala did not compile with the intended legacy compiler. It would be nice to have test infrastructure for these behaviors, of course.

[SolalPirelli]

Does "canonical" have a specific meaning in context here?

[som-snytt]

Thank-you for asking the hard questions. I will answer when I have a canonical answer.

[som-snytt]

The failure is coverage blah blah the single test with legacy compiler tests/run/lazyVals_c3.1.0.scala.

Aggravated by bootstrapped test not taking a single test name, so I'm running the entire suite per cycle. Maybe I'll figure out why the test rig is broken.

[som-snytt]

Realized that I have to use JDK 17 to run that test, stop laughing.

The bigger joke is that bootstrapped test suite crashed my WSL process.

Catastrophic failure
Error code: Wsl/Service/E_UNEXPECTED
Press any key to continue...

[som-snytt]

The error when trying to select tests is

[error] `test / test` tasks in a Scala.js project require `test / fork := false`.

[som-snytt]

After running the successful suite, I only just noticed:

Tossing in options is unexpected. Why isn't it part of the test suite itself?

[som-snytt]

Really any test relying on legacy compilation should be excluded from coverage; there aren't many, so I'll look at why more tests don't fail.

[som-snytt]

I was too "lazy" to check for more tests to exclude, viz,

Test 'tests/run/lazyVals_c3.0.0.scala' failed with output:                      
java.lang.ClassNotFoundException: Test

so obviously I wish I'd just implemented my previous suggestion.

[som-snytt]

Why I'm not a fan of "list of things everyone must continually edit".

If my test can ask to be excluded from Scala-js scalajs: --skip then surely it can do the same for coverage or any facility.

[SolalPirelli]

looks nice now, can we future-proof it?

[SolalPirelli]

any way we can reuse https://github.com/dotty-staging/dotty/blob/main/compiler/src/dotty/tools/dotc/config/ScalaSettingsProperties.scala#L13 to avoid introducing another place that needs changing when we bump the minimum JDK?

[SolalPirelli]

Suggested change

	* `A_1_c3.2.0.scala` in group 1 is compiled by 3.2.0 under canonical JDK 17.
	* `A_1_c3.2.0.scala` in group 1 is compiled by 3.2.0 under the default JDK.

the exact version doesn't really matter here and this comment will become obsolete otherwise, IMHO