Loosen gmxapi stopsignaler test
Gmxapi stopsignaler test was checking whether simulations were stopped
on the step after a stop signal was set. This is true for the legacy
implementation, but only due to the exact order of instructions. The
StopSignalHandler does not guarantee to stop simulations at the next
NS step after a signal has been set, it promises to stop a simulation
at the next NS step after a signal has been communicated. This change
loosens the criterion to reflect this.
Further, this change
* Changes the numberOfTimesCalled() function to timeElapsedSinceStart(),
also including a different treatment of the call before the restraint
is ever called, to avoid misunderstandings.
* Adds a comment to explain the expectations validated by this test.
Fixes #3397
Change-Id: I2a3805a14c03d0ee12ebd21b5863c5243b1c3671