Fix Selenium test. Format of mOTP popup had changed.

The selfservice popup that appears when a mOTP token has been enrolled
says 'Serial number' instead of 'serial'. The test accepts both formats
now.

Change-Id: Ia0b9e874b2b2dbf62a1a212f6ccabf425d3578e3
Merge "Fix Selenium test. Format of mOTP popup had changed."