Replace stretchr/testify with an internal test suite (#981)

As recommended, an `internal/assert` package was added with a reduced set of assertions. All tests were then refactored to use the internal assertions. When more complex assertions were used, they have been rewritten using logic and the simplified assertions.

Fancy formatting for failures was omitted. The `internal/assert/assertions.diff` function could be overwritten for better formatting. That is where diff libraries are used in other test suites.

Refs: #872

Co-authored-by: Alex Mikitik <alex.mikitik@oracle.com>

errors_test.go

@@ -7,8 +7,8 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/pelletier/go-toml/v2/internal/assert"
 	"github.com/pelletier/go-toml/v2/unstable"
-	"github.com/stretchr/testify/assert"
 )
 
 //nolint:funlen