Check yarn gets lazy loader if it's not installed via npm

tests/options/NVM_LAZY_LOAD adds lazy loader for external yarn

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+#!/bin/sh
+source ../common.sh
+
+# Node.js version to install
+node_version=v5.11.0
+
+# Load zsh-nvm and install Node.js in subshell
+(load_zsh_nvm && nvm install "$node_version" && [[ "$(node --version)" == "$node_version" ]]) || die "node wasn't installed"
+
+# Check node isn't available
+[[ "$(node --version)" != "$node_version" ]] || die "node shouldn't be available $(node --version)"
+
+# Set NVM_LAZY_LOAD to true
+export NVM_LAZY_LOAD=true
+
+# Create placeholder yarn
+yarn() { echo yarn }
+
+# Check yarn is a placeholder function
+[[ $(which yarn) != *"_zsh_nvm_load"* ]] || die "yarn should be a placeholder function"
+
+# Load zsh-nvm
+load_zsh_nvm
+
+# Check yarn is a lazy load function
+[[ $(which yarn) == *"_zsh_nvm_load"* ]] || die "yarn should be a lazy load function"