[lit] Include total elapsed time in xunit output

The time= attribute can also be used for the testsuites tag. While this
attribute appears to be ignored by Jenkins (https://github.com/jenkinsci/junit-plugin/blob/bab34bcc96154a494f8c371953efe06d45813f67/src/main/java/hudson/tasks/junit/SuiteResult.java#L202),
it is still useful if you manually inspect the xml file.

Reviewed By: yln

Differential Revision: https://reviews.llvm.org/D84230