Add Gradle DSL unit test

Verify that hovers and code completion work correctly in *.gradle.kts
build files.

Author: fwcd

server/build.gradle

@@ -97,6 +97,11 @@ task copyPropertiesToTestWorkspace(type: Copy) {
     into file('src/test/resources/additionalWorkspace')
 }
 
+task copyPropertiesToDSLTestWorkspace(type: Copy) {
+    from "$rootDir/gradle.properties"
+    into file('src/test/resources/kotlinDSLWorkspace')
+}
+
 task fixFilePermissions(type: Exec) {
     // When running on macOS or Linux the start script
     // needs executable permissions to run.
@@ -133,7 +138,7 @@ run {
 }
 
 test {
-    dependsOn copyPropertiesToTestWorkspace
+    dependsOn copyPropertiesToTestWorkspace, copyPropertiesToDSLTestWorkspace
 
     testLogging {
         events 'failed'

server/src/test/kotlin/org/javacs/kt/GradleDSLScriptTest.kt

@@ -0,0 +1,22 @@
+package org.javacs.kt
+
+import org.junit.Test
+import org.junit.Assert.assertThat
+import org.hamcrest.Matchers.*
+import org.eclipse.lsp4j.MarkedString
+
+class GradleDSLScriptTest : SingleFileTestFixture("kotlinDSLWorkspace", "build.gradle.kts") {
+    @Test fun `edit repositories`() {
+        val completions = languageServer.textDocumentService.completion(completionParams(file, 7, 13)).get().right!!
+        val labels = completions.items.map { it.label }
+
+        assertThat(labels, hasItem("repositories"))
+    }
+
+    @Test fun `hover plugin`() {
+        val hover = languageServer.textDocumentService.hover(textDocumentPosition(file, 4, 8)).get()!!
+        val contents = hover.contents.left.first().right
+
+        assertThat(contents, equalTo(MarkedString("kotlin", "fun PluginDependenciesSpec.kotlin(module: String): PluginDependencySpec")))
+    }
+}

server/src/test/resources/kotlinDSLWorkspace/.gitignore

@@ -0,0 +1,2 @@
+# Auto-generated by :server:copyPropertiesToDSLTestWorkspace
+gradle.properties

server/src/test/resources/kotlinDSLWorkspace/build.gradle.kts

@@ -0,0 +1,9 @@
+plugins {
+    // TODO: Currently not possible, see https://github.com/gradle/gradle/issues/9830
+    // kotlin("jvm") version "$kotlinVersion"
+    kotlin("jvm") version "1.3.50"
+}
+
+repositories {
+    jcenter()
+}

server/src/test/resources/kotlinDSLWorkspace/settings.gradle.kts

@@ -0,0 +1 @@
+rootProject.name = "Kotlin DSL Workspace"