Fix hardcoded path separator

src/diagnostics.test.ts

@@ -105,7 +105,7 @@ test("addDiagnostic writes temporary diagnostics and flushDiagnostics moves them
     // Check that we logged that the database directory doesn't exist yet
     // and that we moved one diagnostic.
     checkExpectedLogMessages(t, logger.messages, [
-      `but the database at ${testDbDir}/actions does not exist yet`,
+      `but the database at ${path.join(testDbDir, "actions")} does not exist yet`,
       "Moving 1 diagnostic(s) to their respective databases.",
     ]);
   }));