Fix broken `DataProtocol.hexString` test utility

[simonjbeaumont]

Motivation:

The test code has utility extensions for converting bytes to and from hex strings. However, the hexString computed property only returned half the bytes it should because it uses Array.init(unsafeUnitializedCapacity:_:) and set the array count to the number of bytes, rather than double the number of bytes, which is the required number of hexadecimal characters in the resulting string.

Modifications:

• Return the correct count in array initializer in DataProtocol.hexString.
• Add trivial test to roundtrip Data through the two hex string utilities.

Result:

DataProtocol.hexString: String now returns a hex string for all the bytes.