Skip encoding conversion fallback tests on newer macOS

Where

```
iconv("幸福", from = "UTF-8", to = "ISO8859-1", sub = "Unicode")
```

returns "??" instead of "<U+5E78><U+798F>"

tests/testthat/helper-locale.R

@@ -115,3 +115,20 @@ encodings <- function(bytes = FALSE) {
   out
 }
 
+# On some plaftorms iconv doesn't create unicode markers when encoding
+# fails. Instead it transliterates to "?" characters.
+skip_if_no_utf8_marker <- function() {
+  skip <- tryCatch(
+    expr = {
+      out <- iconv("幸福", from = "UTF-8", to = "ISO8859-1")
+      !is_string(out) || !grepl("<", out)
+    },
+    error = function(...) {
+      TRUE
+    }
+  )
+
+  if (skip) {
+    skip("No UTF-8 marker with this version of libiconv.")
+  }
+}

tests/testthat/test-deprecated.R

@@ -14,6 +14,7 @@ test_that("type_of() returns correct type", {
 })
 
 test_that("Unicode escapes are converted to UTF8 characters in env_names()", {
+  skip_if_no_utf8_marker()
   local_lifecycle_silence()
 
   with_non_utf8_locale({

tests/testthat/test-encoding.R

@@ -1,4 +1,5 @@
 test_that("can roundtrip symbols in non-UTF8 locale", {
+  skip_if_no_utf8_marker()
   with_non_utf8_locale({
     expect_identical(
       as_string(sym(get_alien_lang_string())),
@@ -15,6 +16,7 @@ test_that("Unicode escapes are always converted to UTF8 characters on roundtrip"
 })
 
 test_that("Unicode escapes are always converted to UTF8 characters with env_names()", {
+  skip_if_no_utf8_marker()
   with_non_utf8_locale({
     env <- child_env(empty_env())
     env_bind(env, !! get_alien_lang_string() := NULL)

tests/testthat/test-env-binding.R

@@ -395,6 +395,7 @@ test_that("env_poke() doesn't warn when unrepresentable characters are serialise
 
     expect_no_warning(env_poke(e, nm, NA))
 
+    skip_if_no_utf8_marker()
     nms <- env_names(e)
     expect_equal(Encoding(nms), "UTF-8")
   })

tests/testthat/test-nse-defuse.R

@@ -226,6 +226,7 @@ test_that("can capture missing argument", {
 })
 
 test_that("serialised unicode in `:=` LHS is unserialised", {
+  skip_if_no_utf8_marker()
   nms <- with_latin1_locale({
     exprs <- exprs("\u5e78" := 10)
     names(exprs)