Add some more tests for config validation, and fix some gaps (#274)

I set these tests up in #270, but I realized there's a lot more we could
test unrelated to that PR. In this commit I add some more tests. They're
not really exhaustive yet, but they did catch a few bugs, which I fixed:
- we weren't validating the package-name if you do set it (only if we
guess it)
- If you omitted `generated`, you would try to write generated code to
the directory containing `genqlient.yaml`, which makes no sense; now we
default to `generated.go`. In the real world it's probably good to set
explicitly, but it's actually very convenient in tests that we don't
have to, and maybe in small projects too.

generate/testdata/invalid-config/InvalidCasing.yaml

@@ -1,3 +1,4 @@
+package: invalidConfig
 casing:
   enums:
     MyType: bogus

generate/testdata/invalid-config/InvalidOptional.yaml

@@ -0,0 +1,2 @@
+package: invalidConfig
+optional: bogus

generate/testdata/invalid-config/InvalidPackage.yaml

@@ -0,0 +1 @@
+package: bogus-package-name