tests/qapi-schema: Clean up positive test for conditionals

tests/qapi-schema: Clean up positive test for conditionals

Union TestIfUnion is conditional on macros TEST_IF_UNION and
TEST_IF_STRUCT.  It uses TestIfEnum, which is conditional on macro
TEST_IF_ENUM.  If TEST_IF_UNION and TEST_IF_STRUCT are defined, but
TEST_IF_ENUM isn't, the generated code won't compile.

Command test-if-cmd is conditional an macros TEST_IF_CMD and
TEST_IF_STRUCT, and uses TestIfEnum.  Similar issue.

Event TEST_IF_EVENT is conditional an macros TEST_IF_EVT and
TEST_IF_STRUCT, and uses TestIfEnum.  Similar issue.

Replace the uses of TestIfEnum in the latter two by str.

TestIfUnion is now TestIfEnum's only user.  Change TestIfEnum's
condition to TEST_IF_UNION.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230316071325.492471-12-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message corrected]