Fix failures on test coverage CI

[yutannihilation]

(Fix #4446 (comment))

Tests in tests/testthat/test-prohibited-functions.R needs to parse R files to do static analysis. But, files are not always available, so this test might fail and covr::coverage() is the case.

ggplot2/tests/testthat/test-prohibited-functions.R

Lines 49 to 51 in bb8f960

	test_that("list up R files properly", {
	expect_true(length(R_files) > 0)
	})

This pull requests let the test skip on covr::coverage(), and also on CRAN for safety.

[thomasp85]

Thanks!