Fix #341
Fix #341

Go 1.7 subtests are called directly from testing.tRunner, not from the
test that contains the t.Run call. Because of this, the call stack
doesn't contain a function starting with Test, Benchmark, or Example,
causing assert.CallerInfo() to run off the end of the call stack and
return nil.

Look for testing.tRunner explicitly to solve this problem, and return
the list of callers if we do run off the call stack so that if there's a
similar problem in the future we still get an inflated stack trace
rather than no trace at all.

yxd-hde/testify,stretchr/testify,yxd-hde/testify,stretchr/testify,dvln/testify