Avoid && in stopifnot() conditions

[MichaelChirico]

Identified by lintr::conjunct_test_linter().

Compare:

x = TRUE
y = FALSE
# Error: x && y is not TRUE
stopifnot(x, y)
# Error: y is not TRUE

The latter gives a much friendlier failure message.

Alternatively, we could use the R>=4 feature (happy to make the change):

stopifnot("Both x and y must be TRUE" = x && y)
# Error: Both x and y must be TRUE

Note that the code works on R<4, just that it gives the earlier, less-friendly failure message.