Speedup create_test_acc_csv (as only used for unit tests)

Fixes #963 by speeding up the create_test_acc_csv.R function. The writing of the file was by far the slowest component and I have now replaced this by data.table::fwrite.

No need to mention this in changelog as it does not affect GGIR behaviour for the user.

Checklist before merging:

[ ] Existing tests still work (check by running the test suite, e.g. from RStudio).
[x] Added tests (if you added functionality) or fixed existing test (if you fixed a bug).
[ ] Clean code has been attempted, e.g. intuitive object names and no code redundancy.
[ ] Documentation updated:
- [ ] Function documentation
- [ ] Chapter vignettes for GitHub IO
- [ ] Vignettes for CRAN
[x] Corresponding issue tagged in PR message. If no issue exist, please create an issue and tag it.
[ ] Updated release notes in inst/NEWS.Rd with a user-readable summary. Please, include references to relevant issues or PR discussions.
[ ] Added your name to the contributors lists in the DESCRIPTION file, if you think you made a significant contribution.
[ ] GGIR parameters were added/removed. If yes, please also complete checklist below.

If NEW GGIR parameter(s) were added then these NEW parameter(s) are :

[ ] documented in man/GGIR.Rd
[ ] included with a default in R/load_params.R
[ ] included with value class check in R/check_params.R
[ ] included in table of vignettes/GGIRParameters.Rmd with references to the GGIR parts the parameter is used in.
[ ] mentioned in NEWS.Rd as NEW parameter

If GGIR parameter(s) were deprecated these parameter(s) are:

[ ] documented as deprecated in man/GGIR.Rd
[ ] removed from R/load_params.R
[ ] removed from R/check_params.R
[ ] removed from table in vignettes/GGIRParameters.Rmd
[ ] mentioned as deprecated parameter in NEWS.Rd
[ ] added to the list in R/extract_params.R with deprecated parameters such that these do not produce warnings when found in old config.csv files.

wadpac / GGIR

Speedup create_test_acc_csv (as only used for unit tests) #1108