[test][lldb-vscode] Relax assertion to allow multiple compile units returned.

I don't think the intent of this test is to make sure we only have one compile unit; it's to make sure request_compileUnits returns something sensible. Relax the test case to just make sure that the main source file is one of the compile units returned, even if there are others.

Fixes llvm.org/pr49418.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D138344