testsuite: update flux-tree-helper.py for new OutputFormat constructor

[grondo]

Problem: The flux.util.OutputFormat class in flux-core will get an improved constructor where headings becomes an optional keyword argument, but this will break the PerfOutputFormat class in t/scripts/flux-tree-helper.py

Try the new-style constructor arguments in PerfOutputFormat. If that fails, fall back to the old style so that either works for the flux-tree-helper.py test script.

This PR will need to be merged to avoid a future PR in flux-core from breaking the flux-tree tests in flux-sched.

[codecov[bot]]

Codecov Report

Merging #985 (62b053c) into master (01ba6d7) will increase coverage by 0.0%. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #985   +/-   ##
======================================
  Coverage    74.1%   74.1%           
======================================
  Files          78      78           
  Lines        9325    9325           
======================================
+ Hits         6910    6913    +3     
+ Misses       2415    2412    -3     

Impacted Files	Coverage Δ
qmanager/modules/qmanager_callbacks.cpp	75.0% <0.0%> (+1.2%)	:arrow_up:

[grondo]

Thanks!