devel/R-cran-gargle: Update to Update to 1.6.0

• Only needs RUN_DEPENDS since it doesn't require compilation.
• Add TEST_DEPENDS
• Use own do-test because the cran.mk's --no-build-vignettes causes error. (from commit "print/R-cran-knitr: Fix test", 8c03e7b5dd6c )

ChangeLog: https://github.com/r-lib/gargle/releases/tag/v1.6.0
PR: 289267