Migrate to vdiffr 1.0 - Githubissues

https://github.com/r-lib/vdiffr/releases/tag/v1.0.0

Also fixes the CRAN check issues (via https://github.com/rstudio/thematic/pull/105/files#diff-7e3c4b817fe01c351c2be5baa52deb4e6ffc210feabcc9d513847a7156cdf5daR6)

Dear maintainer,

Please see the problems shown on
<https://cran.r-project.org/web/checks/check_results_thematic.html>.

Please correct before 2021-06-23 to safely retain your package on CRAN.

checking tests ... [1s/2s] ERROR
  Running ‘testthat.R’ [1s/2s]
Running the tests in ‘tests/testthat.R’ failed.
Complete output:
  > library(testthat)
  > library(thematic)
  >
  > test_check("thematic")
  Error in get(S3[i, 1L], mode = "function", envir = parent.frame()) :
    object 'print_plot' of mode 'function' was not found
  Calls: test_check ... lapply -> FUN -> eval -> eval -> assignInNamespace -> get
  Execution halted

rstudio / thematic

Migrate to vdiffr 1.0 #105