Adjacency Matrix test coverage increased by adding test line

[akshatahuja1]

Brief description of what is fixed or changed

Increased the test coverage for the adjacency matrix implementation of graph data structure, by adding tests corresponding to the add_edge method.

[codecov[bot]]

Codecov Report

Merging #460 (ffc5f35) into master (f82f52d) will decrease coverage by 0.012%. The diff coverage is n/a.

@@              Coverage Diff              @@
##            master      #460       +/-   ##
=============================================
- Coverage   98.588%   98.575%   -0.013%     
=============================================
  Files           29        29               
  Lines         3754      3792       +38     
=============================================
+ Hits          3701      3738       +37     
- Misses          53        54        +1     

Impacted Files	Coverage Δ
pydatastructs/trees/heaps.py	97.536% <0.000%> (-0.512%)	:arrow_down:
...datastructs/linear_data_structures/linked_lists.py	98.930% <0.000%> (-0.268%)	:arrow_down:
pydatastructs/graphs/algorithms.py	99.501% <0.000%> (-0.002%)	:arrow_down:
pydatastructs/linear_data_structures/__init__.py	100.000% <0.000%> (ø)
pydatastructs/linear_data_structures/algorithms.py	99.765% <0.000%> (+0.024%)	:arrow_up:
pydatastructs/graphs/adjacency_matrix.py	100.000% <0.000%> (+2.083%)	:arrow_up:

[czgdp1807]

Codecov shows that the changes are decreasing the coverage.

[akshatahuja1]

I made the changes by restoring the previous tests. I have pushed those changes with the commit titled 'Tests added for adjacency matrix'.

[czgdp1807]

Thanks @akshatahuja1