mock: remove unnecessary fmt.Sprintf in assert.Fail

[alexandear]

Summary

The PR simplifies implementations of AssertNumberOfCalls, AssertCalled, AssertNotCalled functions by removing redundant fmt.Sprintf.

Changes

Remove fmt.Sprintf call inside assert.Fail.

Motivation

assert.Fail already formats msg with arguments, no need to add fmt.Sprintf, see it's declaration:

func Fail(t TestingT, failureMessage string, msgAndArgs ...interface{})