Use testthat 3e

[yutannihilation]

Now that vdiffr doesn't depend on context() (c.f., #4503), it's possible to migrate to testthat 3e.

• Remove context()
• expect_is() -> expect_s3_class() or expect_type()
• expect_equivalent() -> expect_equal(ignore_attr = TRUE)
• Let expect_equal() ignore the environments of quosures
• Ignore multiple warnings as expect_warning() now captures only one warning
• Skip the test about stringsAsFactors on R >=4.0

[yutannihilation]

This should be ready now.

[thomasp85]

Thanks! took the liberty of merging myself so the test setup is updated ASAP

[yutannihilation]

Thanks for merging!