Import-related mdtests aren't isolated #15027
Description
When testing #15026 I got spurious test failures that look like the different test cases within an mdtest file aren't as isolated as they should be:
At commit 65da139, the Relative - Dotted test fails when you test the entire file:
$ cargo test -p red_knot_python_semantic -- mdtest__import_relative
[snip]
relative.md - Relative - Dotted
crates/red_knot_python_semantic/resources/mdtest/import/relative.md:39 unexpected error: [unresolved-import] "Cannot resolve import `.foo.bar.baz`"
crates/red_knot_python_semantic/resources/mdtest/import/relative.md:41 unmatched assertion: revealed: Literal[42]
crates/red_knot_python_semantic/resources/mdtest/import/relative.md:41 unexpected error: 1 [revealed-type] "Revealed type is `Unknown`"
but pass when you run that test by itself:
$ env MDTEST_TEST_FILTER="relative.md - Relative - Dotted" cargo test -p red_knot_python_semantic -- mdtest__import_relative
Renaming the files in that one test case to be unique within the mdtest file (c5fd3c6) also causes test to pass for both invocations.