Regression on code changes and message changes

[rnveach]

Identified in PR checkstyle/checkstyle#4903 ,

If a PR makes changes to Check logic and changes to a existing message, normal regression will display too many differences as all will be centered around change in message and it will be impossible/hard to discern if any where from code changes.

In this situation, we should force check's message to a specific message for both base and patch runs so only the differences in the code is examined.
Example of configuration for the PR mentioned:

<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
          "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
          "http://www.puppycrawl.com/dtds/configuration_1_3.dtd">

<module name="Checker">
    <property name="charset" value="UTF-8"/>
    <property name="severity" value="warning"/>
    <property name="haltOnException" value="false"/>

    <module name="TreeWalker">
        <module name="SummaryJavadoc">
            <property name="forbiddenSummaryFragments" value="^@return the *|^This method returns |^A [{]@code [a-zA-Z0-9]+[}]( is a )"/>
            <message key="summary.first.sentence" value="First sentence of Javadoc is missing an ending period."/>
        </module>
    </module>
</module>

Notice the message is only for summary.first.sentence as that was what was modified in the PR.