Extend the JUnit report

[toxa81]

This PR is adding extra information to the element of JUnit report. In particular, it adds the following text information:

• working directory
• last 20 lines of stdout
• last 20 lines of stderr

Use case: get useful information quickly while browsing the test matrix in the gitlab UI.

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.93%. Comparing base (3adb203) to head (65e185e).

Files with missing lines	Patch %	Lines
reframe/frontend/reporting/__init__.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3702   +/-   ##
========================================
  Coverage    91.93%   91.93%           
========================================
  Files           62       62           
  Lines        13786    13802   +16     
========================================
+ Hits         12674    12689   +15     
- Misses        1112     1113    +1     

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