api/client: intermittent test failure

[rogpeppe]

This error is occasionally observed from the api/client package when running tests:

--- FAIL: TestRulesetService/WatchRuleset/Cancel (0.00s)
      require.go:1209: 
                Error Trace:    rulesets_test.go:430
                Error:          Should be zero, but was {0xc000222de0 <nil>}
                Test:           TestRulesetService/WatchRuleset/Cancel