Do not spend an instance slot on a relaunch

[SimonCropp]

InnerLaunch killed the tool already showing a pair and then asked MaxInstance
whether it was allowed to launch. Once the per process counter was spent the
answer was no - so a re-failing test closed its own diff window and opened
nothing in its place, reporting TooManyRunningDiffTools with the move sent as
processId null.

The count is meant to bound how many tools are open at once, and a replacement
does not raise that number. KillIfNotMdi now reports whether it actually closed
something, and a launch that is replacing skips the check. An MDI tool closes
nothing, so it is not replacing and still counts.

MaxInstance gains an internal ResetCount, because the counter is process wide
and never reset, so a limit means nothing definite in a test otherwise.

Worth knowing for anyone else writing one of these: MaxInstance reads
DiffEngine_MaxInstances before the app domain value, and that variable persists
per user - it is set to 5 on this machine, which is why the test sets it per
process rather than calling MaxInstancesToLaunch alone. My first version did
call it alone, and silently ran with a limit of 5.