add test for skipLinesMatching setting of license header step

Author: abelk2

testlib/src/main/resources/license/SkipLines.test

@@ -0,0 +1,9 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN" "https://checkstyle.org/dtds/configuration_1_3.dtd">
+
+<module name="Checker">
+    <module name="ThisIsNotARealCheckstyleConfigFolks">
+        <property name="goodAdvice" value="dontTryItAnakin"/>
+        <property name="adviceGiver" value="generalKenobi"/>
+    </module>
+</module>

testlib/src/main/resources/license/SkipLinesHasLicense.test

@@ -0,0 +1,13 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN" "https://checkstyle.org/dtds/configuration_1_3.dtd">
+<!--
+  -- This is a fake license header.
+  -- All rights reserved.
+  -->
+
+<module name="Checker">
+    <module name="ThisIsNotARealCheckstyleConfigFolks">
+        <property name="goodAdvice" value="dontTryItAnakin"/>
+        <property name="adviceGiver" value="generalKenobi"/>
+    </module>
+</module>

testlib/src/test/java/com/diffplug/spotless/generic/LicenseHeaderStepTest.java

@@ -122,6 +122,13 @@ void should_remove_header_when_empty() throws Throwable {
 				.test(getTestResource("license/HasLicense.test"), getTestResource("license/MissingLicense.test"));
 	}
 
+	@Test
+	void should_skip_lines_matching_predefined_pattern() throws Throwable {
+		StepHarness.forStep(LicenseHeaderStep.headerDelimiter("<!--\n  -- This is a fake license header.\n  -- All rights reserved.\n  -->", "^(?!<!--|\\s+--).*$")
+			.withSkipLinesMatching("(?i)^(<\\?xml[^>]+>|<!doctype[^>]+>)$").build())
+			.testResource("license/SkipLines.test", "license/SkipLinesHasLicense.test");
+	}
+
 	private String licenceWithAddress() {
 		return "Copyright &#169; $YEAR FooBar Inc. All Rights Reserved.\n" +
 				" *\n" +