MDL-68846 behat: Avoid asserting AM/PM strings. They are not cross-os

MDL-68846 behat: Avoid asserting AM/PM strings. They are not cross-os

The exists/contains steps of behat are case-sensitive, in the other
side, depending of the OS and locales, time functions are not 100%
consistent. See https://bugs.php.net/bug.php?id=76378 for example.

So only solutions are:

1) Create a specific new step performing the checks case insensitively.
2) Avoid making AM / PM part of the checks.

Here we have opted by the 2nd approach, it's not critical part to check.