Let the install snapshots fail on an unrepaired path

The snapshot transforms normalized backslashes out of the diagnostics.
That was added to make the tests pass on Windows before the paths
install_executable() builds were repaired, and it outlived its reason:
with those paths repaired, a backslash reaching one of these messages is
the WSL regression itself, and normalizing it away meant no snapshot
could ever catch it.

Only the fixture directory is still normalized, and only in the value
being matched rather than in the message, because withr::local_tempdir()
and repair_path() disagree about a duplicated separator.

Author: jgabry

tests/testthat/test-model-compile.R

@@ -581,7 +581,6 @@ test_that("a leftover backup doesn't unwind a compile when warnings are errors",
       # Separators are normalized first: on Windows the backup path arrives as
       # "<dir>\exe-old-1234", since tempfile() joins with a backslash.
       transform = function(lines) {
-        lines <- gsub("\\\\", "/", lines)
         for (dir in unique(c(model_dir, repair_path(model_dir)))) {
           lines <- gsub(dir, "<dir>", lines, fixed = TRUE)
         }

tests/testthat/test-utils.R

@@ -240,8 +240,10 @@ exe_path_transform <- function(fixture) {
     repair_path(fixture$dir),
     gsub("\\\\", "/", fixture$dir)
   ))
+  # Deliberately not normalizing separators in the message itself:
+  # install_executable() repairs the paths it builds, so a backslash reaching a
+  # diagnostic is a regression these snapshots should catch, not hide.
   function(lines) {
-    lines <- gsub("\\\\", "/", lines)
     for (dir in dirs) {
       lines <- gsub(dir, "<dir>", lines, fixed = TRUE)
     }