SI-7750 Test case for fixed spurious existential feature warning

In 2.11.0-M8, the enclosed test issued the following feature
warning:

    sandbox/t7750.scala:7: warning: the existential type LazyCombiner[_$1,_$2,_$3] forSome { type _$1; type _$2; type _$3 <: Growable[_$1] with Sizing }, which cannot be expressed by wildcards,

This went way in 2.11.0-RC1 after we turned off the problematic
attempt to infer existential bounds based on the bounds of the
corresponding type parameters.

Review by @soc