Restore global tracing options in the parameterized tag tests

The parameterized publisher/consumer and basic.get tag tests set the
process-global UseRoutingKeyAsOperationName and TracingOptions
.UsePublisherAsParent to their theory inputs and never restored them.
The (false, false) case left both false, so a later test in the same
process that assumed the defaults could fail depending on execution
order. The PlainOperationNames scope added for the new tests covered
only UseRoutingKeyAsOperationName, and only on those tests.

Add a TracingOptionsScope that snapshots both options and restores them
on dispose, and use it at the top of all six parameterized tag tests so
each leaves the global tracing state as it found it.