Supports limited unquoting to make it easier to generate readable failures
within a function or for loop. See quasi_label for more details.
expected
Single numeric value to compare.
label
Used to customise failure messages. For expert use only.
expected.label
Used to customise failure messages. For expert
use only.
...
For expect_equal() and expect_equivalent(), passed on
compare(), for expect_identical() passed on to identical().
Used to control the details of the comparison.