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-file.bats

@@ -2,8 +2,8 @@
 
 load test_helper
 
-setup() {
-  mkdir -p "$PYENV_TEST_DIR"
+_setup() {
+  mkdir -p "${PYENV_TEST_DIR}"
   cd "$PYENV_TEST_DIR"
 }