Arm backend: Add support for BN fusing during QAT (#10967)

Makes it possible to annotate patterns with more than two operators.
This allows us to annotate patterns: conv -> bn and conv -> bn -> relu
to be able to fold away BN after training in QAT. Also adds support for
QAT in Tester class.

Signed-off-by: Oscar Andersson <oscar.andersson@arm.com>