Spinal Tap has reunited — this time in a Los Angeles court to challenge the French studio Vivendi S.A. for millions in profits from the mockumentary “This Is Spinal Tap.” Christopher Guest, Michael McKean and Rob Reiner on Tuesday joined a federal lawsuit filed last year by bandmate and Spinal Tap co-creator Harry Shearer. Shearer, who also voices numerous characters on “The Simpsons,” sued Vivendi S.A. and its subsidiary StudioCanal in October alleging that the companies fraudulently withheld profits from the 1984 film. …