Improve test coverage

This commit improves the test coverage to add strict template assertion
for well known use cases. This allows to catch obvious template issues.

initializr-generator/src/test/resources/project/java/standard/DemoApplication.java.gen

@@ -0,0 +1,12 @@
+package com.example;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class DemoApplication {
+
+	public static void main(String[] args) {
+		SpringApplication.run(DemoApplication.class, args);
+	}
+}