support transform argument from testthat in app$expect_*()

[maxheld83]

testthat's expect_snapshot() has a neat transform argument, which lets you run, well, transformations on values before they are submitted to comparison.

transform
Optionally, a function to scrub sensitive or stochastic text from the output. Should take a character vector of lines as input and return a modified character vector as output.

(source https://testthat.r-lib.org/reference/expect_snapshot.html?q=transform#ref-usage)

Would be nice if the relevant shinytest2 functions also supported this.