Add datetime unit tests

[gowerc]

Closes #121

Main change is adding the timezone to the print out of datetime variables to make it explicit what they are showing as otherwise it leads to some confusing scenarios where the output diff shows the same character values even though the underlying numeric has changed.

[github-actions]

Code Coverage Summary

Filename                Stmts    Miss  Cover    Missing
--------------------  -------  ------  -------  --------------------------
R/ascii_tables.R          115       8  93.04%   10, 154, 164, 169-172, 238
R/cast_variables.R         49       0  100.00%
R/diffdf.R                208      18  91.35%   373-390, 417
R/generate_keyname.R       10       1  90.00%   16
R/identify.R              152       8  94.74%   283-290
R/is_different.R           52       0  100.00%
R/issuerows.R              40       0  100.00%
R/issues.R                 17       1  94.12%   51
R/misc_functions.R         34       2  94.12%   9, 13
R/print.R                  20       0  100.00%
TOTAL                     697      38  94.55%

Results for commit: d681de8

Minimum allowed coverage is 80%

♻️ This comment has been updated with latest results

[github-actions]

Unit Tests Summary

  1 files   13 suites   6s ⏱️
 55 tests  53 ✅ 2 💤 0 ❌
589 runs  580 ✅ 9 💤 0 ❌

Results for commit d681de8.

♻️ This comment has been updated with latest results.

[kieranjmartin]

Looks good. I considered whether in the case where we had the same char value but different time zones if we wanted to show the difference in the values (e.g. display the "actual time" under a reference time, but I think this approach works.