use BATS_FILE_TMPDIR in test/plugin test

use global setup with bats with possibility to add specific test file _setup
use readlink in helper if realpath is not working as expected

test/version.bats

@@ -6,8 +6,8 @@ create_version() {
   mkdir -p "${PYENV_ROOT}/versions/$1"
 }
 
-setup() {
-  mkdir -p "$PYENV_TEST_DIR"
+_setup() {
+  mkdir -p "${PYENV_ROOT}"
   cd "$PYENV_TEST_DIR"
 }
 
@@ -78,4 +78,4 @@ OUT
   assert_output <<OUT
 3.3.3
 OUT
-}
+}