xunit timestamp should be ISO8601

[oost]

JUnit schema requires ISO8601 timestamps (https://github.com/windyroad/JUnit-Schema/blob/master/JUnit.xsd#L165).

Current implementation of the xunit reporter uses RFC1123 date stamps via toUTCString() (https://github.com/tapjs/tap-mocha-reporter/blob/master/lib/reporters/xunit.js#L65).

This could be fixed by using toISOString() instead.

Note: this bug exists in the mocha reporter as well
Note 2: this would be a breaking change