test: make npm tests work on prerelease node versions

PR-URL: #4960
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

deps/npm/test/common-tap.js

@@ -18,6 +18,8 @@ process.env.npm_config_cache = exports.npm_config_cache = npm_config_cache
 process.env.npm_config_userconfig = exports.npm_config_userconfig = path.join(__dirname, 'fixtures', 'config', 'userconfig')
 process.env.npm_config_globalconfig = exports.npm_config_globalconfig = path.join(__dirname, 'fixtures', 'config', 'globalconfig')
 process.env.random_env_var = 'foo'
+// suppress warnings about using a prerelease version of node
+process.env.npm_config_node_version = process.version.replace(/-.*$/, '')
 
 var bin = exports.bin = require.resolve('../bin/npm-cli.js')
 var chain = require('slide').chain