We can all remember how Trump boasted that he would readily accept the Mueller team's invitation to testify (even under oath....but that wouldn't make any difference anyway.)
Now, the mood has drastically changed.....
IF Trump refuses to testify, the only other choice for Mueller is to ask one of the grand juries to issue a subpoena....
When that happens, Trump's legal team will try to have the SCOTUS address the issue....and, based on the Watergate history, the SCOTUS would most likely refuse to let Trump off the hook (a Trump phrase regarding his indicted buddy, Flynn.)
So, the only other choice that Trump has, is to face the Mueller team, and
a. Lie (suicidal)
b. Tell the truth (also suicidal)...or
c. Plead the Fifth (politically ALSO suicidal.)
Fun times ahead.