Add support for async test methods to testgen

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Overwrite assert_not_reached() in test classes

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add expectation_failed() to test classes

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>