Add semanticdb test for #11693

[tanishiking]

#11693

I'm not sure which commit changed the semanticdb behavior, but it looks like already somehow fixed. Added regression tests by adding a code to toplevel.scala and run:

sbt
> scala3-compiler-bootstrapped/Test/runMain dotty.tools.dotc.semanticdb.updateExpect

[bishabosha]

Thank you for opening this, unfortunately, the problem described in #11693 is still not fixed, as can be seen on the first line of the test file in this PR:
https://github.com/lampepfl/dotty/blob/78c9bc58d330cd2945c26d98fb9ddc215fe93a11/tests/semanticdb/expect/toplevel.expect.scala#L1

The problem is that there is any occurrence of the /*<-_empty_::toplevel$package.*/ definition, rather than only it appearing next to a given - this happens wherever the first top level definition in the file exists

[tanishiking]

this happens wherever the first top level definition in the file exists

Ah, I see, thank you for the feedback!
I'll close this PR for now, since it may take some time to fix the underlying problem.