Sensitivity and specificity of PCR based test will be very high. However, an antigen test was mentioned in the press conference and that could be more problematical, as will the antibody test, because they will require certain levels for detection.
Setting cut off points for positive/negative will depend on why they are testing. For the antibody test, if they are testing for presence of antibodies to enable people to return to work they will not want false positives as this could put those returning in danger.
Surveillance testing to estimate numbers in the population are not so critical.
This whole area of sensitivity, specificity and likelihood is fascinating.