[Tests] use $NVM_DIR instead of relative paths

test/fast/Unit tests/nvm_has_system_node

@@ -1,15 +1,14 @@
 #!/bin/sh
 
-cleanup () {
-  rm ../../../v0.1.2/node
-  rmdir ../../../v0.1.2
-}
-die () { echo $@ ; exit 1; }
-
 . ../../../nvm.sh
 
-mkdir ../../../v0.1.2
-touch ../../../v0.1.2/node
+cleanup () {
+  rm -rf "${NVM_DIR}/v0.1.2/node"
+}
+die () { echo $@ ; exit 1; }
+
+mkdir "${NVM_DIR}/v0.1.2"
+touch "${NVM_DIR}/v0.1.2/node"
 
 nvm use 0.1.2
 
@@ -26,4 +25,3 @@ if command -v node; then
 else
   ! nvm_has_system_node
 fi
-