Add tests for Number#toPrecision

test/built-ins/Number/prototype/toPrecision/exponential.js

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

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

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

...e/toPrecision/return-abrupt-tointeger-precision-symbol.js

...rototype/toPrecision/return-abrupt-tointeger-precision.js

test/built-ins/Number/prototype/toPrecision/return-values.js

.../Number/prototype/toPrecision/this-is-0-precision-is-1.js

...ototype/toPrecision/this-is-0-precision-is-gter-than-1.js

...type/toPrecision/this-type-not-number-or-number-object.js

...t-ins/Number/prototype/toPrecision/tointeger-precision.js

...s/Number/prototype/toPrecision/undefined-precision-arg.js