Enhance test coverage for derive_var_trtemfl()

[manciniedoardo]

Background Information

It would be great to get {admiral}'s test coverage to 100%. However, this should not come at the expense of development time/effort from {admiral} core developers. As such, I have made a set of issues tagged "Unit Tests" (such as this one for derive_var_trtemfl()) where users who wish to dip their feet into the {admiral} ecosystem can:

• Learn a bit about unit testing for R packages
• Make a meaningful contribution to {admiral}

by adding or modifying unit tests to account for lines not previously covered.

Important note: Given the above, this issue is for newcomers to {admiral} or users who wish to contribute to the package, and should not be taken on by core {admiral} developers.

Definition of Done

• Unit test(s) added/modified to test the lines in red in derive_var_trtemfl():