Drop REPL parse warnings on parse error

[som-snytt]

Fixes #25055

As explained in the comment in Run, REPL does a parse (which may produce diagnostics) and then a compile (which may produce more diagnostics); the parse warnings will be suspended and then moved to the new Run. However, this commit drops those warnings if the parse is erroneous. The errors are issued, and as usual errors have precedence over warnings. Moreover, the warnings are not relevant because the source text must be amended to pass compilation.

The REPL tests are adjusted so that the test text looks the same as a real REPL transcript, where there is a "trailing space" in the prompt but no "leading space" in the diagnostic text. The adjustment makes it easier to paste a transcript to a test.

Have you relied on LLM-based tools in this contribution?

No

How was the solution tested?

New automated tests (including the issue's reproducer, if applicable)

[som-snytt]

That is indeed a lot of seconds.

[info] Test dotty.tools.dotc.IdempotencyTests.idempotency started
failed because test 'CheckPosIdempotency from idempotency/check' timed out      
[error] Test dotty.tools.dotc.IdempotencyTests.idempotency failed: java.lang.AssertionError: Run test failed, but should not, reasons:
[error] encountered 1 test failure(s):
[error]   - test 'CheckPosIdempotency from idempotency/check' timed out, took 487.911 sec

================================================================================
Test Report
================================================================================

1 suites passed, 1 failed, 2 total
    CheckPosIdempotency from idempotency/check failed, test 'CheckPosIdempotency from idempotency/check' timed out


[error]     at dotty.tools.vulpix.ParallelTesting$CompilationTest.checkPass(ParallelTesting.scala:1337)
[error]     at dotty.tools.vulpix.ParallelTesting$CompilationTest.checkRuns(ParallelTesting.scala:1297)
[error]     at dotty.tools.dotc.IdempotencyTests.idempotency(IdempotencyTests.scala:61)
[error]     at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error]     at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
[error]     at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error]     at java.lang.reflect.Method.invoke(Method.java:569)
[error]     ...
[info] Test run dotty.tools.dotc.IdempotencyTests finished: 1 failed, 0 ignored, 1 total, 487.913s

[SolalPirelli]

Nice, and thanks for all the test adjustments!

[SolalPirelli]

(leaving as draft in case you have another change you want to make but feel free to undraft&merge)