Add dynamically-generated case-sensitive test.

tests/test-helpers/app4-both/README.txt

@@ -1 +0,0 @@
-TODO: my mac won't let me create `r/` and `R/` so I can't test this.

tests/test-helpers/app4-both/r/lower.R

@@ -0,0 +1 @@
+lowerHelper <- 123

tests/testthat/test-app.R

@@ -19,6 +19,13 @@ test_that("lower-case helper dir is loaded", {
 })
 
 test_that("app with both r/ and R/ prefers R/", {
+  ## App 4 already has a lower-case r/ directory. Try to create an upper.
+  tryCatch(dir.create("../test-helpers/app4-both/R"),
+           warning=function(w){testthat::skip("File system is not case-sensitive")})
+  writeLines("upperHelper <- 'abc'", file.path("../test-helpers/app4-both/R", "upper.R"))
+
   shiny:::loadHelpers("../test-helpers/app4-both")
-  testthat::skip("Test not yet implemented")
+
+  expect_equal(lowerHelper, 123)
+  expect_equal(upperHelper, "abc")
 })