A fantastic new feature in Summer ’17 release of Salesforce now allows you to define which unit tests can safely be run in parallel!
If you’ve disabled running parallel unit testing in your Salesforce org because of issues with concurrency (i.e. UNABLE_TO_LOCK_ROW exception) then this update is for you! You can still keep running of unit tests in parallel disabled, but you can now optionally indicate which unit tests can be run in parallel.
To do this, modify the annotation on your unit tests to include the following new parameter.
This should drastically speed up running of your unit tests in your Salesforce org! Find more on this in the Salesforce documentation: