Correct test assertion (Fixes issue#61)

Changed Number.isNaN_Boolean.js, String.prototype.endsWith_Fail.js and String.prototype.endsWith_Fail_2.js to test for the correct result. Also removed implicit coercion in a couple of other test files.
Merge pull request #76 from anba/issue-61/wrong-test-assertions

Correct test assertion (Fixes issue#61)