sf:INVALID_OPERATION: INVALID_OPERATION: testLevel of NoTestRun cannot be used in production organizations

UPDATE 9/2019

As of Summer '19 v46.0, force:source:deploy now works when deploying to production, and you'll no longer get that testLevel error message.

You can set the test level using the -l flag and use -u to point to your connected Production org:

sfdx force:source:deploy -m ApexClass:MyClass -l RunLocalTests -u MyProductionOrg

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OLD ANSWER

If you're using force:source:deploy, it doesn't work when deploying to production, which is why those testLevel errors are showing up. As of Spring 19, you can't adjust those parameters (and this is by design as far as I can tell). This limitation can be found on the force:source:deploy documentation:

The force:source:deploy, force:source:retrieve, and force:source:delete commands work on sandboxes, Developer Edition orgs, and trial orgs, but not production orgs. For production orgs, continue to use force:mdapi:deploy and force:mdapi:retrieve.

For a nice trick to deploy from Sandbox to Production, see my answer to this post - I use this on a daily basis now

To deploy to production using the CLI:

Deploy a Class with specific Test Class (ApexClass:MyCless = metadatatype:fileName)

sfdx force:source:deploy -m ApexClass:MyCless -l RunSpecifiedTests -r TestClass -u ConnectedOrg

Deploy Classes with specific Test Classes (comma Separated metadatatype:fileName,metadatatype:fileName)

sfdx force:source:deploy -m ApexClass:MyCless,ApexClass:TestCless,ApexClass:OtherTestClass -l RunSpecifiedTests -r TestClass,OtherTestClass -u ConnectedOrg