[c# epoxy] Improve multiple shutdown test
[c# epoxy] Improve multiple shutdown test

Before, the `Connection_CanBeStoppedMultipleTimes` test was just
asserting that the connection had been shutdown once--not
twice--before asserting.

Now, we assert that both shutdowns have happened.

Closes https://github.com/Microsoft/bond/pull/321

Microsoft/bond,gencer/bond,gencer/bond,chwarr/bond,chwarr/bond,tstein/bond,jdubrule/bond,Microsoft/bond,tstein/bond,chwarr/bond,sapek/bond,tstein/bond,gencer/bond,chwarr/bond,jdubrule/bond,sapek/bond,chwarr/bond,sapek/bond,sapek/bond,Microsoft/bond,jdubrule/bond,tstein/bond,chwarr/bond,jdubrule/bond,sapek/bond,Microsoft/bond,gencer/bond,jdubrule/bond,gencer/bond,tstein/bond,jdubrule/bond,tstein/bond,sapek/bond,Microsoft/bond,Microsoft/bond