tests/run-tests: Consider all tests as native when emit=native is used.

So that they are skipped when running on a target that doesn't support the
native emitter, eg a nanbox build.

tests/run-tests

@@ -435,7 +435,7 @@ def run_tests(pyb, tests, args, base_path="."):
 
         test_basename = test_file.replace('..', '_').replace('./', '').replace('/', '_')
         test_name = os.path.splitext(os.path.basename(test_file))[0]
-        is_native = test_name.startswith("native_") or test_name.startswith("viper_")
+        is_native = test_name.startswith("native_") or test_name.startswith("viper_") or args.emit == "native"
         is_endian = test_name.endswith("_endian")
         is_int_big = test_name.startswith("int_big") or test_name.endswith("_intbig")
         is_bytearray = test_name.startswith("bytearray") or test_name.endswith("_bytearray")