testsuite: utils__executable_path test change dir to lowercase

This test creates a directory named "Bin". This is incompatible
with the run-cross linux qemu driver as the shared folder between
host and qemu target uses the FAT filesystem. Change the directory
name to "bin" as the case does not seem integral to the test.

Issue: gnatcoll-core#10

Author: Aleksandra Pasek

testsuite/tests/utils/executable_path/test.adb

@@ -19,10 +19,10 @@ begin
       Success : Boolean;
    begin
       A.Assert (OS.Is_Regular_File (Exe_Path));
-      if not OS.Is_Directory ("Bin") then
-         Dir.Create_Directory ("Bin");
-         OS.Copy_File (Exe_Path, "Bin", Success, Preserve => OS.Full);
-         Process_Status := OS.Spawn ("Bin/test", Args => (1 .. 0 => null));
+      if not OS.Is_Directory ("bin") then
+         Dir.Create_Directory ("bin");
+         OS.Copy_File (Exe_Path, "bin", Success, Preserve => OS.Full);
+         Process_Status := OS.Spawn ("bin/test", Args => (1 .. 0 => null));
          A.Assert (Process_Status = 0);
       end if;
       Dir.Set_Directory ("..");