[tests] StandaloneRestIntegTestTask should set a minimum maxParallelForks of 1 (elastic#126346) (elastic#128392)

(cherry picked from commit a13a46a)

Author: brianseeders

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/test/rest/RestTestBasePlugin.java

@@ -167,7 +167,7 @@ public void apply(Project project) {
             nonInputSystemProperties.systemProperty(TESTS_FEATURES_METADATA_PATH, () -> featureMetadataConfig.getAsPath());
 
             // Enable parallel execution for these tests since each test gets its own cluster
-            task.setMaxParallelForks(task.getProject().getGradle().getStartParameter().getMaxWorkerCount() / 2);
+            task.setMaxParallelForks(Math.max(1, task.getProject().getGradle().getStartParameter().getMaxWorkerCount() / 2));
             nonInputSystemProperties.systemProperty(TESTS_MAX_PARALLEL_FORKS_SYSPROP, () -> String.valueOf(task.getMaxParallelForks()));
 
             // Disable test failure reporting since this stuff is now captured in build scans