Optimize decodeURI/encodeURI tests (#987)

* Move decimalToHexString into harness instead of duplicating it in multiple files

* Optimize decimalToHexString and support numbers greater than 65535

* Replace alternative decimalToHexString function with include for decimalToHexString.js

* Add decimalToHex2String to return the string representation of a two-digit hex-number

* Replace decimalToHex2String with decimalToPercentHexString to return the percent hex-encoded string of a two-digit hex-number

* Replace two String.fromCharCode calls with a single call

* Further reduce string concatentations in decodeURI[Component] tests

* Remove unnecessary Test262Error error handling in catch-clauses

* Remove try/catch wrappings in decodeURI/encodeURI tests