test-cases: fix race with zig run on C backend tests

Also avoid redundantly doing compile-error checks on multiple targets
for test cases where that is not helpful.

test/cases/compile_errors/invalid_array_elem_ty.zig

@@ -6,6 +6,7 @@ pub export fn entry() void {
 }
 
 // error
-// backend=stage2,llvm
+// target=native
+// backend=stage2
 //
 // :4:1: error: expected type, found fn() type