attribute: clarify xxhash test purpose (#8753)

## Description:

Issue #7616 asks whether `TestIntegrity` is needed in the
`attribute/internal/xxhash` tests because its purpose was not clear.

This PR keeps the test, renames it, and documents what it verifies:
hashing a string prefix and a `uint64` suffix should match hashing the
equivalent byte sequence directly, which depends on `Uint64` using
little-endian encoding.

Closes #7616.

---------

Signed-off-by: Mridankan Mandal <xerontitan90@gmail.com>

Author: RedZapdos123

attribute/internal/xxhash/xxhash_test.go

@@ -11,6 +11,9 @@ import (
 )
 
 func TestIntegrity(t *testing.T) {
+	// Verify hashing a string prefix and a uint64 suffix produces the same
+	// digest as hashing the equivalent byte sequence directly. This ensures
+	// Uint64 uses the expected little-endian byte encoding.
 	data := []byte{'1', '2', 3, 4, 5, 6, 7, 8, 9, 10}
 	h0 := New()
 	want := h0.String(string(data))