testing: replace legacy gopkg.in/check.v1

This commit replaces the legacy `gopkg.in/check.v1` testing dependency
with `github.com/stretchr/testify`.

Closes https://github.com/distribution/distribution/issues/3884.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

vendor/github.com/stretchr/testify/require/doc.go

@@ -0,0 +1,29 @@
+// Package require implements the same assertions as the `assert` package but
+// stops test execution when a test fails.
+//
+// # Example Usage
+//
+// The following is a complete example using require in a standard test function:
+//
+//	import (
+//	  "testing"
+//	  "github.com/stretchr/testify/require"
+//	)
+//
+//	func TestSomething(t *testing.T) {
+//
+//	  var a string = "Hello"
+//	  var b string = "Hello"
+//
+//	  require.Equal(t, a, b, "The two words should be the same.")
+//
+//	}
+//
+// # Assertions
+//
+// The `require` package have same global functions as in the `assert` package,
+// but instead of returning a boolean result they call `t.FailNow()`.
+//
+// Every assertion function also takes an optional string message as the final argument,
+// allowing custom error messages to be appended to the message the assertion method outputs.
+package require