[SUREFIRE-2230] Make failure and error element in XML test report schemas complex instead of simple

**[Michael Osipov](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=michael-o)** opened **[SUREFIRE-2230](https://issues.apache.org/jira/browse/SUREFIRE-2230?redirect=false)** and commented

Because of (bad/legacy) design choices both schemas https://maven.apache.org/surefire/maven-surefire-plugin/xsd/surefire-test-report.xsd and https://maven.apache.org/surefire/maven-failsafe-plugin/xsd/failsafe-test-report.xsd make both elements `failure` and `error` simple where the stack trace is not a nested element, but the body of those elements. This is inconsistent with other elements. With the next major version of Surefire this shall be changed to complex elements with a nested `stackTrace` elements.

This is the outcome of SUREFIRE-2229.

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**Affects:** 3.2.3

**Issue Links:**
- [SUREFIRE-2229](https://issues.apache.org/jira/browse/SUREFIRE-2229) [REGRESSION] SUREFIRE-2224 causes stack trace to be omitted for errors and failures
(_**"is caused by"**_)

Start with the surefire-test-report.xsd and failsafe-test-report.xsd schemas linked in the issue, and review SUREFIRE-2229 for the preceding regression context. Change the failure and error elements to complex elements containing nested stackTrace elements, then verify both schemas represent the intended report structure for the next major version.

Written by the indexing model from the issue text.