contrib/plugins: init with mTLS store example

This reverts commit 2b42901ec7.
Reason for revert: the work required has been achieved.

Change-Id: I917b2f87e436a4e34bec0b788634d949b0236c62

tests/functional/init.sh

@@ -10,7 +10,7 @@ if test -d "$TEST_ROOT"; then
     killDaemon
     rm -rf "$TEST_ROOT"
 fi
-mkdir "$TEST_ROOT"
+mkdir -p "$TEST_ROOT"
 
 mkdir "$NIX_STORE_DIR"
 mkdir "$NIX_LOCALSTATE_DIR"