Build: Fix ability to ignore when no tests are run (#28930)

Running any randomized testing task within Elasticsearch currently fails
if a project has zero tests. This was supposed to be overrideable, but
it was always set to 'fail', and the system property to override was
passed down to the test runner, but never read there. This commit
changes the value of the ifNoTests setting to randomized runner to be
read from system properties and continue to default to 'fail'.

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -558,7 +558,7 @@ class BuildPlugin implements Plugin<Project> {
         return {
             jvm "${project.runtimeJavaHome}/bin/java"
             parallelism System.getProperty('tests.jvms', 'auto')
-            ifNoTests 'fail'
+            ifNoTests System.getProperty('tests.ifNoTests', 'fail')
             onNonEmptyWorkDirectory 'wipe'
             leaveTemporary true
 
@@ -582,8 +582,6 @@ class BuildPlugin implements Plugin<Project> {
             systemProperty 'tests.task', path
             systemProperty 'tests.security.manager', 'true'
             systemProperty 'jna.nosys', 'true'
-            // default test sysprop values
-            systemProperty 'tests.ifNoTests', 'fail'
             // TODO: remove setting logging level via system property
             systemProperty 'tests.logger.level', 'WARN'
             for (Map.Entry<String, String> property : System.properties.entrySet()) {