swift234

[validation-test] More granular FixedPoint tests

`FixedPoint.swift.gyb` generates a massive amount of assertions:
one for each `Int` type (such as `UInt8` and `Int64`) as a source,
to one of each `Int` type as a destination, for each of 27 bit
patterns. By my math that's 8 * 8 * 27 == 1728 assertions.
As a result, when one of those assertions fails, it's difficult to
tell what went wrong.

Split each assertion into its own test case. This makes the test
take a little longer to run, but it produces much more valuable
output when it fails.