Qualify namespaces in tests to avoid library() calls

[nanxstats]

Following the recommendation in R Packages (also tidyverse and tidymodels conventions), this PR removes the extra library() calls from tests/testthat.R and individual tests, and qualify the namespaces explicitly.

The involved namespaces and functions include (might not be exhaustive):

- gsDesign: gsDesign gsSurv gsBoundSummary sfLDOF
- tibble: tibble
- dplyr: filter mutate transmute full_join group_by summarize ungroup
  first last lag lead select arrange desc right_join rename

[LittleBeannie]

Thanks, @nanxstats !