QTest: Fix printing of values of Q_ENUMs

QMetaEnum::key(n) returns the n-th key, which is not necessarily the one
of value n. For the key of value n, we want QMetaEnum::valueToKey(n).

Change-Id: Ic90fe6b1cbe84978a02fffff141bf4a06074917a
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>