Test more UTF-8 characters in transcode (JuliaLang#55580)

Previous test just covered 2-byte UTF8. This one should hit more
branches for ASCII and 3-byte UTF-8.

test/strings/basic.jl

@@ -1392,8 +1392,15 @@ 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
+    # string starting with an ASCII character
+    str_1 = "zβγ"
+    # string starting with a 2 byte UTF-8 character
+    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)
+        @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
 end