tests: fix duplicate and failing test
There were two "testconflictingrequestid" methods. Naturally this isn't
an error in Python. And by our luck, the test was failing.
So we rename the test and fix it to pass.
As part of this, _sendsingleframe() now takes a frame, not a string
describing the frame. This is better because action at a distance can
be confusing.
Differential Revision: https://phab.mercurial-scm.org/D2950
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