changes to run against stage (or other) remote server

For example, this will run against stage - ./node_modules/.bin/intern-client config=tests/intern_server fxaAuthRoot=https://api-accounts.stage.mozaws.net/v1 fxaContentRoot=https://accounts.stage.mozaws.net/ fxaEmailRoot=http://restmail.net fxaOauthApp=https://123done-stage.dev.lcip.org/ fxaProduction=true

NOTE: Not all of the tests, e.g., tests/server/metrics-collector-stderr.js, make sense to run remotely. Those test suites are skipped if fxaProduction=true.
Maybe make this part of intern.config too so that we don't have duplication removing the trailing slash.