readme: Recommend npm 'install-test' instead of 'install & test'
readme: Recommend npm 'install-test' instead of 'install & test'

This reduces the number of sub processes and makes the output a bit
cleaner. The command was introduced in npm 3 (current stable: npm 6).

Change-Id: I01757b36d1bdcadb8957b95239d33ce267cc2028

wikimedia/oojs,wikimedia/oojs