Remove ellipsis dependency + address deprecation warnings

[olivroy]

In favour of rlang since ellipsis is deprecated. https://rlang.r-lib.org/news/index.html#argument-intake-1-0-0

While running tests, I noticed some deprecation warnings, that I took care of

• Remove context(..) in tests (the context is now derived from test file name)
• expect_equivalent(...) -> expect_equal(..., ignore_attr = TRUE)
• aes_string("x", "y") -> aes(.data$x, .data$y) (this required me to use usethis::use_import_from("rlang", ".data") as suggested in ggplot2 in packages vignette
• Remove covr from Suggests, as the test-coverage action installs the package already.
• Removed trailing whitespace

[olivroy]

the test-coverage action can be fixed by following steps outlined in r-lib/actions#834