Even MORE tests for transcode (JuliaLang#55649)

Missed 4 byte UTF-8, transcoding string to string, and transcoding to
signed integers

test/strings/basic.jl

@@ -1398,9 +1398,14 @@ end
     str_2 = "αβγ"
     # string starting with a 3 byte UTF-8 character
     str_3 = "आख"
-    @testset for str in (str_1, str_2, str_3)
+    # string starting with a 4 byte UTF-8 character
+    str_4 = "𒃵𒃰"
+    @testset for str in (str_1, str_2, str_3, str_4)
+        @test transcode(String, str) === str
         @test transcode(String, transcode(UInt16, str)) == str
         @test transcode(String, transcode(UInt16, transcode(UInt8, str))) == str
+        @test transcode(String, transcode(Int32, transcode(UInt8, str))) == str
+        @test transcode(String, transcode(UInt32, transcode(UInt8, str))) == str
         @test transcode(String, transcode(UInt8, transcode(UInt16, str))) == str
     end
 end