fix clipboard on linux

tests/testthat/test-clipboard.R

@@ -1,9 +1,9 @@
 context("clipboard.R")
 
 test_that("clipboard works", {
-  skip_on_os(c("linux", "solaris"))
   t1 <<- AMR::antibiotics # why is the <<- needed? Won't work without it...
   clipboard_export(t1, info = FALSE)
   t2 <- clipboard_import()
+  skip_if(is.null(t1) | is.null(t2), message = "No clipboard content found: skipping.")
   expect_equal(t1, t2)
 })