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/init.bats

@@ -2,7 +2,7 @@
 
 load test_helper
 
-setup() {
+_setup() {
   export PATH="${PYENV_TEST_DIR}/bin:$PATH"
 }