Improve test coverage and standardise test library

[will-molloy]

• Currently the coverage is a low 54%
• Would be nice to also standardise the libraries used (upgrade to junit5/assertj?)

[rabidgremlin]

lol. you read my mind :)

[will-molloy]

@rabidgremlin ill work on this

[rabidgremlin]

Have merged the work to standardize the libraries from #43