Add tests for UniqueConcertList

[bbbbcccd]

Fixes #52

• Fix casing in UniqueConcertList
• Create ConcertBuilder class
• Create TypicalConcerts class
• Add valid name, address, date for typical concerts to CommandTestUtil
• Add tests for UniqueConcertList

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...seedu/address/model/concert/UniqueConcertList.java	80.00%	0 Missing and 1 partial :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...seedu/address/model/concert/UniqueConcertList.java	78.26% <80.00%> (+78.26%)	17.00 <3.00> (+17.00)

... and 3 files with indirect coverage changes