api_v3_SyntaxConformanceTest - Fix hard failure

The arbitrarily generated value of "tax_rate" violates the MySQL precision
constraints. This commit ensures that the arbitrarily chosen values meet
the constraint (while trying to preserve the old series of generated
numbers for most use-cases).
Merge pull request #2610 from totten/master-fix-api-abort

api_v3_SyntaxConformanceTest - Fix hard failure