Fix build for JDK 18 >= EA b21 (#1249)

Ant task `java` with parameter `fork="false"` calls
`java.lang.System.setSecurityManager`,
however tests should not call it,
because as part of work on JEP 411
in JDK 17 it was marked as deprecated
(see https://bugs.openjdk.java.net/browse/JDK-8264713)
and in JDK 18 throws UnsupportedOperationException
unless system property `java.security.manager` set to `allow`
(see https://bugs.openjdk.java.net/browse/JDK-8270380).

org.jacoco.ant.test/src/org/jacoco/ant/CoverageTaskTest.xml

@@ -152,7 +152,7 @@
 		<au:assertFileExists file="${exec.file}"/>
 		<au:assertLogContains text="Target executed"/>
 
-		<java classname="org.jacoco.ant.DumpExecClassNames" classpath="${java.class.path}" failonerror="true">
+		<java classname="org.jacoco.ant.DumpExecClassNames" fork="true" classpath="${java.class.path}" failonerror="true">
 			<arg value="${exec.file}" />
 		</java>
 		<au:assertLogContains text="java/sql/Timestamp"/>

org.jacoco.ant.test/src/org/jacoco/ant/ReportTaskTest.xml

@@ -71,7 +71,7 @@
 	</target>
 
 	<target name="testReportWithSourceButNoDebug">
-		<java classname="org.jacoco.ant.RemoveDebugInfos" classpath="${java.class.path}" failonerror="true">
+		<java classname="org.jacoco.ant.RemoveDebugInfos" fork="true" classpath="${java.class.path}" failonerror="true">
 			<arg value="${org.jacoco.ant.reportTaskTest.classes.dir}/org/jacoco/ant/TestTarget.class" />
 			<arg value="${temp.dir}/TestTarget.class" />
 		</java>
@@ -89,7 +89,7 @@
 	</target>
 
 	<target name="testReportWithSourceDirButNoDebug">
-		<java classname="org.jacoco.ant.RemoveDebugInfos" classpath="${java.class.path}" failonerror="true">
+		<java classname="org.jacoco.ant.RemoveDebugInfos" fork="true" classpath="${java.class.path}" failonerror="true">
 			<arg value="${org.jacoco.ant.reportTaskTest.classes.dir}/org/jacoco/ant/TestTarget.class" />
 			<arg value="${temp.dir}/TestTarget.class" />
 		</java>