How is a positive horror rating handled?
Yes,

Horror tests are an attribute test, which are defined on page 13 of the rules, emphasis mine. 


  Occasionally, an investigator must make a modified attribute
  test. Such tests always list a positive or negative number after the attribute (for example: “Test Strength –1”). This number is always added to the investigator’s attribute before rolling the die. In this way, a skill test with a negative modifier is more likely to fail.