Collocations for testify - verb

falsely, truthfully
The basis for the perjury conviction was he had testified falsely under oath.

publicly
personally

ask sb to, call sb to, call upon sb to, subpoena sb to (esp. AmE)
The president's former aides were called to testify at his trial.

compel sb to, force sb to
agree to, be prepared to, be willing to
refuse to
come forward to
Several witnesses have come forward to testify against her.

be allowed to
be expected to, be scheduled to
take the stand to
He took the stand to testify at the trial of his colleague.

against
She refused to testify against her brother.

before
She was willing to testify before Congress.

for
a Mafia member who was prepared to testify for the authorities

to
A senior officer testified to the existence of police hit squads.
A large number of witnesses testified to the tribunal.

testify in court, testify under oath (esp. AmE)

Testify is used with these nouns as the subject:
witness