skip tests if environment variables not set

[mrustl]

see: https://github.com/r-universe/kwb-r/runs/6801782786?check_suite_focus=true#step:8:610

 ERROR
Running the tests in 'tests/testthat.R' failed.
Last 13 lines of output:
  `expected`: TRUE 
  ── Failure (test-function-nextcloud_url.R:3:3): nextcloud_url() works ──────────
  startsWith(kwb.nextcloud:::nextcloud_url(), "https://") is not TRUE
  `actual`:   FALSE
  `expected`: TRUE 
  ── Failure (test-function-nextcloud_user.R:7:3): nextcloud_user() works ────────
  nchar(result) > 1L is not TRUE
  `actual`:   FALSE
  `expected`: TRUE 
  [ FAIL 5 | WARN 0 | SKIP 0 | PASS 56 ]
  Error: Test failures