Add IDE tests that use trees from TASTY

[Duhemm]

Currently, the IDE tests work as if all the sources for a test were opened in a buffer before the test starts: https://github.com/lampepfl/dotty/blob/5a07d51fc09a854f0c4d43158bc31d3f81288b5d/language-server/test/dotty/tools/languageserver/util/CodeTester.scala#L19-L22

This doesn't accurately represent how the IDE is actually going to be used and we risk missing bugs and regressions. (See for instance: #5222)

We should also have tests where the sources are not opened, but their trees are unpickled from TASTY instead.