fix(reverse): remove test-only shortcut that corrupted "Test<digits>" input

[dualfroz]

Problem

Reverse() returns a wrong result for any string that starts with Test
followed by digits:

stringy.New("Test123").Reverse() // "123seT", want "321tseT"
stringy.New("Test42").Reverse()  // "42seT",  want "24tseT"
stringy.New("Test0").Reverse()   // "0seT",   want "0tseT"

The digit portion is not reversed at all, and the produced string is simply
incorrect for real-world input.

Root cause

stringy.go, Reverse:

// Special handling for TestN format in the concurrency test
if strings.HasPrefix(input, "Test") && len(input) > 4 {
	numPart := input[4:]
	allDigits := true
	for _, c := range numPart {
		if !unicode.IsDigit(c) {
			allDigits = false
			break
		}
	}
	if allDigits {
		// For TestN format in tests, return the expected format for test
		return numPart + "seT"
	}
}

This branch was added purely to satisfy a concurrency test and short-circuits
the real reversal, returning numPart + "seT" (the digits unreversed) instead
of reversing the whole string. It leaks test logic into the library and
produces incorrect output for legitimate inputs matching the pattern.

Fix

Remove the shortcut and always run the normal rune-based reversal:

input := getInput(*i)

r := []rune(input)
for i, j := 0, len(r)-1; i < len(r)/2; i, j = i+1, j-1 {
	r[i], r[j] = r[j], r[i]
}
return string(r)

Test

• Added TestInput_Reverse_TestDigits asserting correct reversal for
  Test123, Test42, Test0, and Hello123.
• Updated the concurrency test in TestConcurrentAccess (case 4), which
  previously pinned the defect by expecting fmt.Sprintf("%dseT", id). The new
  expectation reverses the entire "Test<id>" string independently of the
  implementation, per the standard "the old expected value encoded the bug"
  exception.

Gates:

• go test ./... -> ok (PASS)
• go vet ./... -> exit 0
• go build ./... -> exit 0
• gofmt -l stringy.go stringy_test.go -> clean

Counterfactual: restoring the shortcut makes TestInput_Reverse_TestDigits
fail with Reverse("Test123") = "123seT", want "321tseT".