Add an equivalence test for the optimized Ok() method (#280)

The C++ back end rewrites the per-field Ok() checks of conditional fields
that share a discriminant into a switch, and gates residual arms on the
residual predicate alone. There is no option to emit the unoptimized form,
so this test pins Ok() to an independent reference written directly from
its definition: the structure is complete, and every field's existence is
Known and, when present, the field is Ok().

Every structure with a conditional discriminant in condition.emb, plus
LargeConditionals, DisjunctionConditionals and OuterGuardedUnion from
many_conditionals.emb, is checked at every buffer length from zero to its
full size. The bytes that existence conditions read take every value; the
remaining payload bytes take two fill patterns; the four-byte discriminants
are sampled over every arm, the gaps between arms and the top of the
range, as documented in the test. Each test also asserts the number of
buffers it checked, so the enumeration cannot shrink unnoticed.

Compiled against headers regenerated with earlier generators and the same
schema, the test fails on the master generator (ResidualConditional-,
DisjunctionConditional- and EnumConditionalDiscriminant: a buffer with
`outer != 1` is wrongly rejected) and passes from the guard-demotion fix
onwards.