log contexts for failures in JUnit reporter

[runave]

Description

We are using doctest with JUnit reports in our pipeline and if something goes wrong, then sometimes it's helpful to see the contexts of the failure as well.
I have extended JUnitReporter with log_contexts in my fork, based on the similar functionality of ConsoleReporter - please feel free to accept the pull request.
example failure with contexts:

      <failure message="1 != 1" type="CHECK">
logging.cpp(0):
CHECK( vec[i] != (1 &lt;&lt; i) ) is NOT correct!
  values: CHECK( 1 != 1 )
  logged: current iteration of loop:
          i := 0

      </failure>

[onqtam]

Merged the PR - thanks for the contribution! Closing this issue and will release a new version of doctest hopefully in 1-2 weeks.