Move /src/test to /tests

tests/ui/maximal_mir_to_hir_coverage.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Zmaximal-hir-to-mir-coverage
+// run-pass
+
+// Just making sure this flag is accepted and doesn't crash the compiler
+
+fn main() {
+  let x = 1;
+  let y = x + 1;
+  println!("{y}");
+}