Remove warnings from deprecated atomic operators in autotests

Remove warnings from deprecated atomic operators in autotests

Use QAtomic*::load() and ::store() instead of the deprecated
cast, assignment, and comparison operators. These will be removed
in the near future.

The tests for these particular operators have not been changed,
though, as the change to remove the operators will also remove
the respective tests.

Change-Id: I2f24d18992af0c6e0f487d707218e4e84f4bdd12
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>