Ensure consistent left/right solution brackets for bracketing solvers

[dcourteville]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

cf. SciML/DiffEqBase.jl#1234 & #765

[dcourteville]

Now all solvers pass the zero-tolerance tests except Muller (because it's not an actual bracketing solver) and Falsi (because it's not numerically stable at very low tolerance).