Understand the opinion that not every snag requires a test flight. That is entirely realistic and is normal for the industry. What can be argued is the need for a test flight when primary flight controls are involved along with a continuous stick-shaker and the caution messages that were experienced on the previous three flights, and this goes to communication when writing up snags, (as has already been observed here). PJ2
That's on the company, possibly on some of the trouble shooting trees available to the techs, and who knows what dialogue the company had with their Boeing field support/tech support liaison.
That's an angle of inquiry well worth pursuing.
Had the company maintenance team a gap in their tech manuals?