CBG-4455 do not panic when calling FailNow

[torcolvin]

CBG-4455 do not panic when calling FailNow

Confusingly, require.FailNow's signature is FailNow(t TestingT, failureMessage string, msgAndArgs ...interface{}). There are a few consequences to this, one of which is that expecting failureMessage to be formatted will not happen. The second problem is if the first argument in msgAndArgs is not a format string, then the test harness can panic.

• Created a ruleguard rule to find the misuse of this. This prevents use of % followed by an letter but most "real" uses of % are followed by a space, number, or punctuation. Forcing passing to fmt.Sprintf will cause the compiler to error if the wrong argument types are passed. Updated ruleguard rules in linter to account for new checks, and changed the string to only pick up rules-*.go files so the test files in that directory aren't picked up.
• Fixed existing misuse.

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2898/ (known, unrelated failure)

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