Improve coverage of pynumero mpi block matrix tests

[michaelbynum]

Changes proposed in this PR:

• Improve testing for PyNumero's MPIBlockMatrix

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

Merging #2318 (588c721) into main (536deaa) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2318      +/-   ##
==========================================
+ Coverage   85.24%   85.40%   +0.16%     
==========================================
  Files         619      619              
  Lines       76453    76572     +119     
==========================================
+ Hits        65169    65393     +224     
+ Misses      11284    11179     -105     

Flag	Coverage Δ
linux	82.42% <100.00%> (+0.35%)	⬆️
osx	72.66% <0.00%> (+0.20%)	⬆️
other	82.37% <100.00%> (+0.36%)	⬆️
win	79.46% <0.00%> (+0.22%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyomo/contrib/pynumero/sparse/block_matrix.py	89.96% <ø> (+0.29%)	⬆️
pyomo/contrib/pynumero/sparse/mpi_block_matrix.py	81.23% <100.00%> (+14.46%)	⬆️
pyomo/core/kernel/dict_container.py	98.86% <0.00%> (-1.14%)	⬇️
pyomo/solvers/plugins/solvers/gurobi_direct.py	71.91% <0.00%> (-0.51%)	⬇️
examples/pyomobook/test_book_examples.py	96.80% <0.00%> (-0.41%)	⬇️
pyomo/contrib/appsi/solvers/gurobi.py	82.90% <0.00%> (-0.35%)	⬇️
pyomo/core/kernel/block.py	99.70% <0.00%> (-0.30%)	⬇️
pyomo/contrib/mindtpy/iterate.py	83.56% <0.00%> (+1.82%)	⬆️
... and 1 more

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[jsiirola]

Looks great!