Unify messages about exit codes in StandaloneTests

Some tests in StandaloneTests provide concatenation of standard output
and standard error as a message for the assertion about exit code, while
other tests provide only standard output.  Some use method
de.sormuras.bartholdy.Result#getOutput while others use method
de.sormuras.bartholdy.Result#getOutputLines.

Unify these approaches by replacing the messages with a supplier that
gives a) a human-readable message about exit codes, and b) both standard
output and standard error.

platform-tooling-support-tests/src/test/java/platform/tooling/support/tests/StandaloneTests.java

@@ -69,7 +69,7 @@ void listAllObservableEngines() {
 				.addArguments("engines", "--disable-banner").build() //
 				.run(false);
 
-		assertEquals(0, result.getExitCode(), String.join("\n", result.getOutputLines("out")));
+		assertEquals(0, result.getExitCode(), () -> getExitCodeMessage(result));
 
 		var jupiterVersion = Helper.version("junit-jupiter-engine");
 		var suiteVersion = Helper.version("junit-platform-suite-engine");
@@ -98,7 +98,7 @@ void compile() throws Exception {
 				.addArguments(workspace.resolve("src/standalone/VintageIntegration.java")).build() //
 				.run();
 
-		assertEquals(0, result.getExitCode(), result.getOutput("out") + result.getOutput("err"));
+		assertEquals(0, result.getExitCode(), () -> getExitCodeMessage(result));
 		assertTrue(result.getOutput("out").isEmpty());
 		assertTrue(result.getOutput("err").isEmpty());
 
@@ -327,7 +327,7 @@ private static Result discover(String... args) {
 				.build() //
 				.run(false);
 
-		assertEquals(0, result.getExitCode(), String.join("\n", result.getOutputLines("out")));
+		assertEquals(0, result.getExitCode(), () -> getExitCodeMessage(result));
 		return result;
 	}
 
@@ -349,7 +349,7 @@ void execute() throws IOException {
 				.addArguments("--classpath", "bin").build() //
 				.run(false);
 
-		assertEquals(1, result.getExitCode(), String.join("\n", result.getOutputLines("out")));
+		assertEquals(1, result.getExitCode(), () -> getExitCodeMessage(result));
 
 		var workspace = Request.WORKSPACE.resolve("standalone");
 		var expectedOutLines = Files.readAllLines(workspace.resolve("expected-out.txt"));
@@ -384,7 +384,7 @@ void executeOnJava8() throws IOException {
 				.addArguments("--classpath", "bin").build() //
 				.run(false);
 
-		assertEquals(1, result.getExitCode(), String.join("\n", result.getOutputLines("out")));
+		assertEquals(1, result.getExitCode(), () -> getExitCodeMessage(result));
 
 		var workspace = Request.WORKSPACE.resolve("standalone");
 		var expectedOutLines = Files.readAllLines(workspace.resolve("expected-out.txt"));
@@ -420,7 +420,7 @@ void executeOnJava8SelectPackage() throws IOException {
 				.addArguments("--classpath", "bin").build() //
 				.run(false);
 
-		assertEquals(1, result.getExitCode(), String.join("\n", result.getOutputLines("out")));
+		assertEquals(1, result.getExitCode(), () -> getExitCodeMessage(result));
 
 		var workspace = Request.WORKSPACE.resolve("standalone");
 		var expectedOutLines = Files.readAllLines(workspace.resolve("expected-out.txt"));
@@ -461,6 +461,11 @@ void executeWithJarredTestClasses() {
 				.build() //
 				.run(false);
 
-		assertEquals(1, result.getExitCode(), String.join("\n", result.getOutputLines("out")));
+		assertEquals(1, result.getExitCode(), () -> getExitCodeMessage(result));
+	}
+
+	private static String getExitCodeMessage(Result result) {
+		return "Exit codes don't match. Stdout:\n" + result.getOutput("out") + //
+				"\n\nStderr:\n" + result.getOutput("err") + "\n";
 	}
 }