Display diffs when snapshotting text files

[hadley]

I haven't yet figured out how to test snapshots with snapshots, so you can run this to test interactively

test_that("expect_snapshot_file() show differences for text files", {
  path1 <- withr::local_tempfile(lines = sample(letters[1:10]))
  expect_snapshot_file(path1, "text-diff.txt")
})

Fixes #1593

@schloerke can you please try this out and see how it feels?

[schloerke]

Test

test_that("expect_snapshot_file() show differences for text files", {
  path1 <- withr::local_tempfile(
    lines = jsonlite::toJSON(sample(letters[1:10]))
  )
  expect_snapshot_file(path1, "text-diff.json")
})

testthat v3.2.3

✔ | F W  S  OK | Context
✖ | 1        1 | validate_remotes_order
─────────────────────────────────────────────
Failure (test-validate_remotes_order.R:13:3): expect_snapshot_file() show differences for text files
Snapshot of `path1` to 'validate_remotes_order/text-diff.json' has changed
Run `testthat::snapshot_review('validate_remotes_order/')` to review changes
─────────────────────────────────────────────

══ Results ════════════════════════════════════════════════════════
── Failed tests ─────────────────────────────────────
Failure (test-validate_remotes_order.R:13:3): expect_snapshot_file() show differences for text files
Snapshot of `path1` to 'validate_remotes_order/text-diff.json' has changed
Run `testthat::snapshot_review('validate_remotes_order/')` to review changes

[ FAIL 1 | WARN 0 | SKIP 0 | PASS 1 ]
Error: Test failures

testthat#2205

✔ | F W  S  OK | Context
✖ | 1        1 | validate_remotes_order                                                                                                                       
─────────────────────────────────────────────
Failure (test-validate_remotes_order.R:13:3): expect_snapshot_file() show differences for text files
Snapshot of `path1` has changed.
Differences:
    old                                       | new                                          
[1] ["h","i","d","a","e","g","c","j","b","f"] - ["f","i","a","j","g","b","d","e","c","h"] [1]
* Run `testthat::snapshot_accept('validate_remotes_order/')` to accept the change.
* Run `testthat::snapshot_review('validate_remotes_order/')` to review changes

─────────────────────────────────────────────

══ Results ════════════════════════════════════════════════════════
── Failed tests ─────────────────────────────────────
Failure (test-validate_remotes_order.R:13:3): expect_snapshot_file() show differences for text files
Snapshot of `path1` has changed.
Differences:
    old                                       | new                                          
[1] ["h","i","d","a","e","g","c","j","b","f"] - ["f","i","a","j","g","b","d","e","c","h"] [1]
* Run `testthat::snapshot_accept('validate_remotes_order/')` to accept the change.
* Run `testthat::snapshot_review('validate_remotes_order/')` to review changes


[ FAIL 1 | WARN 0 | SKIP 0 | PASS 1 ]
Error:
! Test failures.

---

Looks great! Thank you!!