Make it easier to extend by simplifying the definition of a test case.

(cherry picked from commit 479beda)

This adds more tests for the avro incompatibility
`reader_field_missing_default_value`. All of these tests are already
passing, while the tests added in the following commit show the change
in behaviour.

(cherry picked from commit 6aabcfa)

This fixes a bug in the avro compatibility check for fields present in
the reader but missing from the writer. In this case the writer and the
reader are compatible only if the reader field has a specified default
value.

The Avro library sets the default value to `GenericDatum()` which has
type `AVRO_NULL` when the default is not specified. This is what we are
trying to detect in this check. However, the check was confusing an
explicit null default for a union-type as a missing value. The former is
represented as `GenericDatum(Union(Null))` and because of the way
`GenericDatum::type()` delegates to the type of the first union leaf
value when `GenericDatum` contains a union, the check thought that the
value was missing, when it was in fact specified as null.

To solve for the above, we also check that the type is not a union type
to be able to detect when the default is not set. (The same check is
used in the Avro library to detect unset default values, for example,
when printing an Avro schema as JSON.)

The caveat is that we are still overly restrictive for fields of type
null, because in that case, the default value will be `GenericDatum()`
regardless of whether the default value was explicitly set to null or
not. This is a limitation of the Avro library's API and could only be
fixed by making breaking changes to the Avro library's API.

Also note that the checks against `reader.leafAt(...)` have been
removed. They were attempting to compare the type of the field against
the type of the default value. This is not necessary because this is
ensured by the Avro library's built-in schema validation done while
parsing the Avro schema.

(cherry picked from commit 3866b1c)