[Tests] use $NVM_DIR instead of relative paths

test/fast/Listing versions/Running "nvm ls 0.2" should display only 0.2.x versions.

@@ -1,11 +1,11 @@
 #!/bin/sh
 
-mkdir ../../../v0.1.3
-mkdir ../../../v0.2.3
-mkdir ../../../v0.20.3
-
 . ../../../nvm.sh
 
+mkdir "${NVM_DIR}/v0.1.3"
+mkdir "${NVM_DIR}/v0.2.3"
+mkdir "${NVM_DIR}/v0.20.3"
+
 die () { echo $@ ; exit 1; }
 
 # The result should contain only the appropriate version numbers.