Provided a function to set the default failure mode.

Further addresses #146, #210 and #211. Also clarified in the
documentation that the test never need include FailureInherits as a
specified failure mode (per the comments on github).
Using FailureContinues as default mode in assertions tests.

Also using t.Parallel() to facilitate detection of race conditions.
Simplified discovery logic
Replaced a few new() with composite literals
Simplified runner
Simplified scope
Removed unnecessary checks when registering registrations

suiteContext will check if it is actually possible to add
the registration