Some small tests for transcode (#55436)

This logic appears to be
[uncovered](https://app.codecov.io/gh/JuliaLang/julia/blob/master/base%2Fstrings%2Fcstring.jl#L232)
although it does have a doctest! Running the tests locally managed to
trigger these functions.

test/strings/basic.jl

@@ -1388,3 +1388,10 @@ end
         end
     end
 end
+
+@testset "transcode" begin
+    str = "αβγ"
+    @test transcode(String, transcode(UInt16, str)) == str
+    @test transcode(String, transcode(UInt16, transcode(UInt8, str))) == str
+    @test transcode(String, transcode(UInt8, transcode(UInt16, str))) == str
+end