to_string_visitor): Do not parenthesize the top-level formula.
* tgbatest/explicit.test, tgbatest/explpro2.test,
tgbatest/explpro3.test, tgbatest/explprod.test,
tgbatest/readsave.test, tgbatest/tgbaread.test,
tgbatest/tripprod.test: Adjust expected output.
* sanity/style.test: Fix regexes to catch an error seen in
tostring.cc.