I am going for a MRI head scan as I'm suspected to have a pituitary tumor. Will they test for all brain abnormalities or only this?

Probably. The MRI head scan should cover everything, unless your provider asked specifically not to. Sometimes the MRI focuses on the pituitary region only, but many times a full brain screening exam is done with the pituitary scan, so if there is anything else happening, this test will probably help.
Brain MRI. When performing a scan to assess pituitary function the whole brain will be imaged - i.e. the doctor will look at other brain structures besides the pituitary.