build: pin file.encoding for the forked test JVMs as well

ParserKeywordsUtilsTest regenerates the .jj in-process and reads it back, so it
depends on the default charset the same way the build does. Surefire forks its
test JVMs and those do not inherit .mvn/jvm.config, leaving them on the platform
default:

    ParserKeywordsUtilsTest.getAllKeywordsUsingJavaCC ... <<< ERROR!
    org.javacc.parser.MetaParseException

Gradle needs no equivalent: its test workers take their encoding from the daemon,
which gradle.properties already pins.

Author: knutwannheden

pom.xml

@@ -569,11 +569,13 @@
                         @{jacocoArgLine} is resolved late, after jacoco:prepare-agent has run.
                         Without it the JaCoCo agent would be dropped and coverage reports empty.
                     -->
+                    <!-- Forked test JVMs do not inherit .mvn/jvm.config, so pin the encoding here too. -->
                     <argLine>
                         @{jacocoArgLine}
                         --add-opens=java.base/java.lang=ALL-UNNAMED
                         --add-opens=java.base/java.util=ALL-UNNAMED
                         -Xmx2G -Xms800m -Xss4m
+                        -Dfile.encoding=UTF-8
                     </argLine>
                 </configuration>
             </plugin>