Add tests to check that genqlient handles covariance (#203)

I didn't realize until today that implementations of GraphQL interfaces
are actually allowed to be covariant: if the interface has a field
`f: T`, then the implementations may have fields `f: U` where `U` is a
subtype of `T` (for example `U` may be an implementation of the
interface `T`, or `U` may be `T!` if `T` is non-nullable. (I thought it
had to be `f: T` exactly.) So I figured I'd add a test and see what
breaks.

Surprisingly, and despite the fact that Go interfaces do *not* allow
covariance, everything... worked? There's at least one place where it's
possible we could ideally use a more specific type [1], but for now I
just wanted to make sure we at least write something that builds and is
vaguely reasonable. Of course I'm not sure if there's anything I've
missed (some day I need to find a fuzzing engine that can fuzz GraphQL).

[1] Specifically, the field
`CovariantInterfaceImplementationRandomItemTopic.Next` might ideally
have type `...NextContentTopic`, not `...NextContent`; we know it's a
topic. This doesn't directly cause covariance problems in Go: the method
`GetNext` still returns `...NextContent` so the interface matches. But
that trick doesn't work for the sibling field `.Related` which is
slice-typed: or rather, we'd need the method to copy the slice to the
correct type. (Not to mention the implemention of any change here would
require a bunch of plumbing because the AST doesn't quite have what we
want.) So it's probably best to just keep this as-is for simplicity and
consistency.

Test plan: make check

generate/testdata/queries/CovariantInterfaceImplementation.graphql

@@ -0,0 +1,23 @@
+query CovariantInterfaceImplementation {
+  randomItem {
+    id
+    next { ...ContentFields }
+    related { ...ContentFields }
+  }
+  root {
+    ...ContentFields
+    ...TopicFields
+    next { ...TopicFields }
+    related { ...TopicFields }
+  }
+}
+
+fragment ContentFields on Content {
+  next { id }
+  related { id }
+}
+
+fragment TopicFields on Topic {
+  next { id }
+  related { id }
+}

generate/testdata/queries/schema.graphql

@@ -84,6 +84,8 @@ interface Content {
   name: String!
   parent: Topic
   url: String!
+  next: Content
+  related: [Content!]
 }
 
 """An object with a duration, like a video."""
@@ -102,6 +104,8 @@ type Article implements Content {
   url: String!
   text: String!
   thumbnail: StuffThumbnail
+  next: Content
+  related: [Content!]
 }
 
 type StuffThumbnail {  # for articles, but let's give the name-generator a hard time.
@@ -117,6 +121,8 @@ type Video implements Content & HasDuration {
   url: String!
   duration: Int!
   thumbnail: Thumbnail
+  next: Content
+  related: [Content!]
 }
 
 type Thumbnail {  # for videos, but let's give the name-generator a hard time.
@@ -133,6 +139,8 @@ type Topic implements Content {
   children: [Content!]!
   videoChildren: [Video!]!
   schoolGrade: String
+  next: Topic
+  related: [Topic!]
 }
 
 input RecursiveInput {