mock: enable parallel testing on internal testsuite

Add t.Parallel() to all package-level tests of the 'mock' package.
Result: faster tests results.

Before:
  $ go test -count=10 ./mock
  ok  	github.com/stretchr/testify/mock	0.631s
After:
  $ go test -count=10 ./mock
  ok  	github.com/stretchr/testify/mock	0.426s

Author: dolmen