Add small test for sparse_block_matrix

[RainerKuemmerle]

No description provided.

[codecov]

Codecov Report

Merging #532 (e205f35) into master (3733a9b) will increase coverage by 0.49%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #532      +/-   ##
==========================================
+ Coverage   45.56%   46.06%   +0.49%     
==========================================
  Files         271      271              
  Lines       10484    10564      +80     
==========================================
+ Hits         4777     4866      +89     
+ Misses       5707     5698       -9     

Impacted Files	Coverage Δ
g2o/core/sparse_block_matrix.hpp	68.65% <100.00%> (+19.81%)	⬆️

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