Issue #13809: kill mutation for AbstarctRootNode

[Kevin222004]

Issue #13809: kill mutation for AbstarctRootNode

Mutation

checkstyle/config/pitest-suppressions/pitest-xpath-suppressions.xml

Lines 3 to 10 in d316b89

	<mutation unstable="true">
	<sourceFile>AbstractRootNode.java</sourceFile>
	<mutatedClass>com.puppycrawl.tools.checkstyle.xpath.AbstractRootNode</mutatedClass>
	<mutatedMethod>iterateAxis</mutatedMethod>
	<mutator>org.pitest.mutationtest.engine.gregor.mutators.experimental.RemoveSwitchMutator_2</mutator>
	<description>RemoveSwitch 2 (case value 2)</description>
	<lineContent>switch (axisNumber) {</lineContent>
	</mutation>

Attributes are not allowed by design:

checkstyle/src/main/java/com/puppycrawl/tools/checkstyle/xpath/AbstractRootNode.java

Lines 73 to 75 in a7fa83a

	public String getAttributeValue(NamespaceUri namespace, String localPart) {
	throw throwUnsupportedOperationException();
	}

[Kevin222004]

I am not finding any query in which attribute axis is used as a root just like I created a test for an ancestor-or-self axis
#13771.

[romani]

No attributes on root.
Root is

checkstyle/src/main/java/com/puppycrawl/tools/checkstyle/utils/TokenUtil.java

Lines 315 to 317 in a7fa83a

	public static boolean isRootNode(DetailAST ast) {
	return ast.getType() == TokenTypes.COMPILATION_UNIT;
	}

[rdiachenko]

lgtm

[rdiachenko]

rebased

[romani]

@Kevin222004 , please resolve conflict