Add tests for Number#toExponential (#664)

test/built-ins/Number/prototype/toExponential/infinity.js

test/built-ins/Number/prototype/toExponential/nan.js

test/built-ins/Number/prototype/toExponential/prop-desc.js

...onential/return-abrupt-tointeger-fractiondigits-symbol.js

...e/toExponential/return-abrupt-tointeger-fractiondigits.js

...built-ins/Number/prototype/toExponential/return-values.js

.../prototype/toExponential/this-is-0-fractiondigits-is-0.js

...totype/toExponential/this-is-0-fractiondigits-is-not-0.js

...pe/toExponential/this-type-not-number-or-number-object.js

...umber/prototype/toExponential/tointeger-fractiondigits.js

...umber/prototype/toExponential/undefined-fractiondigits.js