added JenkinsShellCallRule class

test/groovy/util/JenkinsShellCallRule.groovy

@@ -0,0 +1,38 @@
+package util
+
+import com.lesfurets.jenkins.unit.BasePipelineTest
+import org.junit.rules.TestRule
+import org.junit.runner.Description
+import org.junit.runners.model.Statement
+
+class JenkinsShellCallRule implements TestRule {
+
+    final BasePipelineTest testInstance
+
+    String shell = ""
+
+    JenkinsShellCallRule(BasePipelineTest testInstance) {
+        this.testInstance = testInstance
+    }
+
+    @Override
+    Statement apply(Statement base, Description description) {
+        return statement(base)
+    }
+
+    private Statement statement(final Statement base) {
+        return new Statement() {
+            @Override
+            void evaluate() throws Throwable {
+
+                testInstance.helper.registerAllowedMethod("sh", [String.class], {
+                    command -> 
+					command = command.replaceAll(/\s+/," ").trim()
+					     shell += "$command \n"
+                })
+
+                base.evaluate()
+            }
+        }
+    }
+}