Add Helper() method to CollectT like testing.T as we intend to add
Helper() to the assert.TestingT interface.

…ests

Add Helper() method in internal mocks and assert.CollectT

Co-authored-by: Olivier Mengué <dolmen@cpan.org>

Shortcut in assert.JSONEq once we have validated that 'expected' is
valid JSON, and 'actual' is the exact same string.

Shortcut in assert.YAMLEq once we have validated that 'expected' is
valid YAML, and 'actual' is the exact same string.

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

…esting

mock: enable parallel testing on internal testsuite

Cleanup runtime use of stdlib's testing internals which was required for older
Go versions.

Note: we are still using testing.RunTests in the suite's test suite for
now.

…f-testing-internals

suite: cleanup use of 'testing' internals at runtime

…mpty-notempty

assert: check test failure message for Empty and NotEmpty

$ go mod edit -dropexclude=github.com/stretchr/testify@v1.8.0 -exclude=github.com/stretchr/testify@v1.8.4
$ go mod tidy

See stretchr/objx#140

deps: fix dependency cycle with objx (again)

Document the assert.Empty rules more comprehensively. This exposes our
quirks to the user to avoid wrong expectations.

Add many many many more test cases that document edges cases and will allow
to catch breaking changes and avoid regressions.