coverage: Migrate tests/run-coverage into tests/coverage

tests/coverage/test_harness.rs

@@ -0,0 +1,10 @@
+// Verify that the entry point injected by the test harness doesn't cause
+// weird artifacts in the coverage report (e.g. issue #10749).
+
+// compile-flags: --test
+
+#[allow(dead_code)]
+fn unused() {}
+
+#[test]
+fn my_test() {}