Use create_hook helper

Use extracted create_hook helper in tests for:

- exec
- hooks
- rehash
- version-name
- version-origin
- which

test/exec.bats

@@ -44,15 +44,13 @@ OUT
 }
 
 @test "carries original IFS within hooks" {
-  hook_path="${RBENV_TEST_DIR}/rbenv.d"
-  mkdir -p "${hook_path}/exec"
-  cat > "${hook_path}/exec/hello.bash" <<SH
+  create_hook exec hello.bash <<SH
 hellos=(\$(printf "hello\\tugly world\\nagain"))
 echo HELLO="\$(printf ":%s" "\${hellos[@]}")"
 SH
 
   export RBENV_VERSION=system
-  RBENV_HOOK_PATH="$hook_path" IFS=$' \t\n' run rbenv-exec env
+  IFS=$' \t\n' run rbenv-exec env
   assert_success
   assert_line "HELLO=:hello:ugly:world:again"
 }