Include stderr output to make the test pass

Author: kyleconroy

internal/endtoend/testdata/vet_failures/exec.json

@@ -0,0 +1,3 @@
+{
+  "command": "vet"
+}

internal/endtoend/testdata/vet_failures/stderr.txt

@@ -0,0 +1,6 @@
+query.sql: GetAuthor: no-pg: invalid engine: postgresql
+query.sql: ListAuthors: no-pg: invalid engine: postgresql
+query.sql: CreateAuthor: no-pg: invalid engine: postgresql
+query.sql: CreateAuthor: only-one-param: too many parameters
+query.sql: DeleteAuthor: no-pg: invalid engine: postgresql
+query.sql: DeleteAuthor: no-delete: don't use delete statements