qapi: Use generated TestStruct machinery in tests

Commit d88f5fd and friends first introduced the various test-qmp-*
tests in 2011, with duplicated hand-rolled TestStruct machinery,
to make sure the qapi visitor interface was tested.  Later, commit
4f193e3 in 2013 added a .json file for further testing use by the
files, but without consolidating any of the existing hand-rolled
visitors.  And with four copies, subtle differences have crept in,
between the tests themselves (mainly whitespace differences, but
also a question of whether to use NULL or "TestStruct" when
calling visit_start_struct()) and from what the generator produces
(the hand-rolled versions did not cater to partially-allocated
objects, because they did not have a deallocation usage).

Of course, just because the visitor interface is tested does not
mean it is a sane interface; and future patches will be changing
some of the visitor contracts.  Rather than having to duplicate
the cleanup work in each copy of the TestStruct visitor, and keep
each hand-rolled copy in sync with what the generator supplies, we
might as well just test what the generator should give us in the
first place.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1446791754-23823-2-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

tests/qapi-schema/qapi-schema-test.out

@@ -92,6 +92,7 @@ object EventStructOne
 object ForceArrays
     member unused1: UserDefOneList optional=False
     member unused2: UserDefTwoList optional=False
+    member unused3: TestStructList optional=False
 enum MyEnum []
 object NestedEnumsOne
     member enum1: EnumOne optional=False
@@ -100,6 +101,10 @@ object NestedEnumsOne
     member enum4: EnumOne optional=True
 enum QEnumTwo ['value1', 'value2']
     prefix QENUM_TWO
+object TestStruct
+    member integer: int optional=False
+    member boolean: bool optional=False
+    member string: str optional=False
 object UserDefA
     member boolean: bool optional=False
     member a_b: int optional=True