call to testing.T.Helper() slows down execution

[funny-falcon-at-joomcode]

Currently every assertion function calls to testing.T.Helper() unconditionally.
testing.T.Helper() collects function name from stack trace, and it is quite expensive operation.
And go 1.12 collects traces even slower than go 1.11, therefore tests became to be slower.

There is no need to call testing.T.Helper() if assertion is satisfied, therefore I propose to move call to Helper to failing case instead of unconditional call.

[MaxInertia]

I just switched our tests from the standard func TestX(t *testing.T) to testify suites func (suite *MyTestSuite) TestX() and noticed the tests are taking considerably longer. If this is the source of that increased time it would be excellent to have this changed.

[bboreham]

Should be improved by golang/go#38834, once the next version of Go comes out.