Test argument and variable nullability

Compare library with JS reference implementation in various nullability
cases. This patch uses JS script to build various schemas and queries
and execute them to generate luatest script that verifies that this
library has the same behavior.

To generate script, run
```bash
cd ./tests/integration/codegen/fuzzing_nullability
npm install
node ./generate.js > ../../fuzzing_nullability_test.lua
```

Prior to this patch, several nullability behavior issues were fixed.
There is some case that isn't yet fully covered by our library, refer
to [1] for updates.

Follow [2] for more test cases.

Based on @no1seman PR [3].

1. #62
2. #63
3. #59

Author: DifferentialOrange

.gitignore

@@ -24,7 +24,7 @@ coverage_result.txt
 .DS_Store
 .vscode
 luacov.*.out*
-/node_modules
+**/node_modules
 /package-lock.json
 *.mo
 .history

.luacheckrc

@@ -8,6 +8,7 @@ include_files = {
 }
 exclude_files = {
     '.rocks',
+    'test/integration/fuzzing_nullability_test.lua',
 }
 new_read_globals = {
     box = { fields = {

Makefile

@@ -3,7 +3,7 @@ SHELL := /bin/bash
 .PHONY: .rocks
 .rocks: graphql-scm-1.rockspec Makefile
 		tarantoolctl rocks make
-		tarantoolctl rocks install luatest 0.5.5
+		tarantoolctl rocks install luatest 0.5.7
 		tarantoolctl rocks install luacov 0.13.0
 		tarantoolctl rocks install luacheck 0.26.0
 

test/helpers.lua

@@ -0,0 +1,36 @@
+local types = require('graphql.types')
+local schema = require('graphql.schema')
+local parse = require('graphql.parse')
+local validate = require('graphql.validate')
+local execute = require('graphql.execute')
+
+local helpers = {}
+
+helpers.test_schema_name = 'default'
+
+function helpers.check_request(query, query_schema, mutation_schema, directives, opts)
+    opts = opts or {}
+    local root = {
+        query = types.object({
+            name = 'Query',
+            fields = query_schema or {},
+        }),
+        mutation = types.object({
+            name = 'Mutation',
+            fields = mutation_schema or {},
+        }),
+        directives = directives,
+    }
+
+    local compiled_schema = schema.create(root, helpers.test_schema_name, opts)
+
+    local parsed = parse.parse(query)
+
+    validate.validate(compiled_schema, parsed)
+
+    local rootValue = {}
+    local variables = opts.variables or {}
+    return execute.execute(compiled_schema, parsed, rootValue, variables)
+end
+
+return helpers

test/integration/codegen/fuzzing_nullability/README.md

@@ -0,0 +1,10 @@
+To install dependencies, run
+```bash
+npm install
+```
+
+To generate test file, run
+```bash
+node generate.js > ../../fuzzing_nullability_test.lua
+```
+in this directory.