Enable parallel testing for almost all tests in packages 'assert' and
'require' by calling t.Parallel() as the first line of the test.
A few tests are incompatible and will be fixed separately. They are
marked with a FIXME.
Incompatible tests: TestFileExists, TestNoFileExists, TestDirExists,
TestNoDirExists.
Before:
$ go test -count=10 ./assert ./require
ok github.com/stretchr/testify/assert 7.575s
ok github.com/stretchr/testify/require 1.501s
After:
$ go test -count=10 ./assert ./require
ok github.com/stretchr/testify/assert 1.703s
ok github.com/stretchr/testify/require 1.245s
