domestic violence under WA state law, i filed a police report against my spouse do i have to testify against them

The DA will expect you to testify. You can say you won't at which point he/she can call you to the stand and the judge will force you to answer. Most of the time if the victim doesn't want to testify however the DA doesn't want to bother. Depends on the extent of any injuries and your past history.

Not too long ago (within the last year) a Seattle city councilmember was arrested and charged with domestic violence for allegedly grabbing his wife.

The case was eventually dismissed because the wife invoked her Fifth Amendment right. She claimed that what she said in police reports the night of the husband's arrest and what her testimony today would be may conflict each other leading to criminal prosecution against her.

Without the wife's testimony, the prosecutor had no admissible evidence.

Here's a link to a newspaper's article regarding that case: http://seattletimes.nwsource.com/html/localnews/2004127722_mciver16m.html .

You may want to retain your own attorney and discuss what options may be available to you.