missing_key.js does some inserts and commands without checking them

coll.insert({x: 1, z: 1});
coll.insert({y: 1, z: 1});
db.adminCommand({enableSharding: 'testDb'});

If these fail, this leads to harder-to-diagnose failures later on. We should use
assert.writeOK() on the insers and assert.commandWorked on the command.