Start moving testing infra as test fixtures

This commit applies the `java-test-fixtures` to the project modules and
starts moving common infrastructure to the testFixtures configuration.

See gh-217

build.gradle

@@ -29,6 +29,7 @@ subprojects {
 
 configure(moduleProjects) {
 	apply plugin: 'java-library'
+	apply plugin: 'java-test-fixtures'
 	apply plugin: 'org.springframework.graphql.compiler'
 
 	java {

spring-graphql-test/build.gradle

@@ -17,6 +17,7 @@ dependencies {
 	compileOnly 'com.google.code.findbugs:jsr305'
 	compileOnly 'org.jetbrains.kotlinx:kotlinx-coroutines-core'
 
+	testImplementation(testFixtures(project(":spring-graphql")))
 	testImplementation 'org.junit.jupiter:junit-jupiter'
 	testImplementation 'org.assertj:assertj-core'
 	testImplementation 'org.mockito:mockito-core'