[gui-test] add an optional prepare gant script to setup environment

This script is needed to perform a custom shell script to setup environment before running tests, e.g. to start X server for UI tests.

build/scripts/pre_tests_in_community.gant

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2000-2018 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+includeTargets << new File("tests_in_community.gant")
+includeTool << gant.tools.Execute
+
+/**
+ * Configures environment by running a shell script delegating by system property "gant.prepare.test.script" and than runs an default target
+ * in tests_in_community.gant
+ */
+
+target(prepare: "Prepare tests environment") {
+  def uiScript = System.getProperty("gant.prepare.test.script")
+  echo message: 'Preparing tests environment... '
+  echo message: "  Running shell script \"$uiScript\""
+  execute.shell(uiScript)
+  "default"()
+}
+
+setDefaultTarget(prepare)