Number of utilities for writing gradle integration tests (#32282)

These are collected from a number of open PRs and are required to
improove existing and write more readable future tests.
I am extracting them to their own PR hoping to be able to merge and use
them sooner.

Author: alpar-t

buildSrc/src/test/java/org/elasticsearch/gradle/test/GradleIntegrationTestCase.java

@@ -1,6 +1,11 @@
 package org.elasticsearch.gradle.test;
 
+import org.gradle.testkit.runner.GradleRunner;
+
 import java.io.File;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 public abstract class GradleIntegrationTestCase extends GradleUnitTestCase {
 
@@ -13,4 +18,47 @@ protected File getProjectDir(String name) {
         return new File(root, name);
     }
 
+    protected GradleRunner getGradleRunner(String sampleProject) {
+        return GradleRunner.create()
+            .withProjectDir(getProjectDir(sampleProject))
+            .withPluginClasspath();
+    }
+
+    protected File getBuildDir(String name) {
+        return new File(getProjectDir(name), "build");
+    }
+
+    protected void assertOutputContains(String output, String... lines) {
+        for (String line : lines) {
+            assertOutputContains(output, line);
+        }
+        List<Integer> index = Stream.of(lines).map(line -> output.indexOf(line)).collect(Collectors.toList());
+        if (index.equals(index.stream().sorted().collect(Collectors.toList())) == false) {
+            fail("Expected the following lines to appear in this order:\n" +
+                Stream.of(lines).map(line -> "   - `" + line + "`").collect(Collectors.joining("\n")) +
+                "\nBut they did not. Output is:\n\n```" + output + "\n```\n"
+            );
+        }
+    }
+
+    protected void assertOutputContains(String output, String line) {
+        assertTrue(
+            "Expected the following line in output:\n\n" + line + "\n\nOutput is:\n" + output,
+            output.contains(line)
+        );
+    }
+
+    protected void assertOutputDoesNotContain(String output, String line) {
+        assertFalse(
+            "Expected the following line not to be in output:\n\n" + line + "\n\nOutput is:\n" + output,
+            output.contains(line)
+        );
+    }
+
+    protected void assertOutputDoesNotContain(String output, String... lines) {
+        for (String line : lines) {
+            assertOutputDoesNotContain(line);
+        }
+    }
+
 }