[Tests] use a newer node version for this test to avoid flakiness

Author: ljharb

test/slow/nvm uninstall/Running 'nvm uninstall' with incorrect file permissions fails nicely

@@ -6,7 +6,7 @@ die () { echo "$@" ; exit 1; }
 \. ../../../nvm.sh
 
 # Version to install/uninstall
-NVM_TEST_VERSION=5.10.1
+NVM_TEST_VERSION=15.5.0
 
 # Make sure it's not already here
 [ -e ../../../$NVM_TEST_VERSION ] && rm -R ../../../$NVM_TEST_VERSION