Restore some commented-out test cases for replset edge cases.

These were commented-out in f91a41d054595f43374a13e63afac120458a43f7
along with some test reorganization. Reinstate them, and update them
for test-suite changes since then.
Merge pull request #150 from nelhage/replset-tests

Restore some commented-out test cases for replset edge cases.