ci: make sure npm is installed in tests

Also bump node version to 10.
13 jobs for fix-npm-in-tests in 18 minutes and 13 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed py2.7-min-postgres #55678

00:06:16

passed py2.7-pypi-postgres #55679

00:06:15

passed py3.6-min-mysql #55684

00:04:19

passed py3.6-min-postgres #55676

00:04:47

passed py3.6-min-sqlite #55672

00:05:03

passed py3.6-pypi-mysql #55675

00:06:39

passed py3.6-pypi-postgres #55733

00:05:33

passed py3.6-pypi-sqlite #55681

00:05:53

failed py3.6-min-mysql #55674

00:07:14

failed py3.6-pypi-postgres #55677

00:03:37

failed py3.6-pypi-postgres #55682

00:02:43

failed py3.6-pypi-sqlite #55673

00:03:28

 
  Coverage
passed global-coverage #55680

00:01:05

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