Add tests for InsurancePlansManager.java

[RezwanAhmed123]

There is not enough tests to ensure robustness of the insurance plan manager.

This will not allow us to catch regression bugs easily when new features are implemented.

Let's add some important basic tests to ensure regression catching is easier in the future.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...du/address/logic/commands/AddInsuranceCommand.java	0.00%	1 Missing :warning:
...address/logic/commands/DeleteInsuranceCommand.java	0.00%	1 Missing :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...s/model/person/insurance/InsurancePlanFactory.java	72.72% <ø> (+45.45%)	5.00 <0.00> (+3.00)
.../model/person/insurance/InsurancePlansManager.java	78.37% <100.00%> (+21.23%)	15.00 <2.00> (+6.00)
...du/address/logic/commands/AddInsuranceCommand.java	21.42% <0.00%> (ø)	3.00 <0.00> (ø)
...address/logic/commands/DeleteInsuranceCommand.java	22.22% <0.00%> (ø)	3.00 <0.00> (ø)