Add gradle compatibility tests; fix compatibility with 5.0

plugin/build.gradle.kts

@@ -1,16 +1,17 @@
 plugins {
-    id("com.github.johnrengelman.shadow") version "4.0.0"
     `java-gradle-plugin`
     `kotlin-dsl`
     `maven-publish`
     kotlin("kapt") version embeddedKotlinVersion
+    id("com.github.johnrengelman.shadow") version "4.0.0"
+    id("org.ysb33r.gradletest") version "2.0-rc.4"
 }
 
 group = "org.nixos"
 version = "1.0.0-SNAPSHOT"
 
 repositories {
-    mavenCentral()
+    jcenter()
 }
 
 dependencies {
@@ -31,4 +32,33 @@ gradlePlugin {
             implementationClass = "org.nixos.gradle2nix.Gradle2NixPlugin"
         }
     }
-}
+}
+
+tasks {
+    gradleTest {
+        versions("5.0", "5.1.1", "5.2.1", "5.3.1")
+        kotlinDsl = true
+    }
+
+    gradleTestGenerator {
+        dependsOn(shadowJar)
+        doLast {
+            file(gradleTest.get().initScript).bufferedWriter().use { out ->
+                out.appendln("""
+                    initscript {
+                        dependencies {
+                            classpath fileTree('file:${buildDir.absolutePath}/libs'.toURI()) {
+                                include '*.jar'
+                            }
+                """.trimIndent())
+
+                out.appendln("""
+                        }
+                    }
+
+                    apply plugin: org.nixos.gradle2nix.Gradle2NixPlugin
+                """.trimIndent())
+            }
+        }
+    }
+}