Bugfix for fi mode of AVOID_NULL_CHECK

### What's done:
 * Added isAssignment utility function to check if a line has assignment operator ('=') using simple regex
 * Added a check for assignment operator when deciding if to put run block or not
 * Added tests
(#1293)

diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/smoke/DiktatSmokeTestBase.kt

@@ -311,7 +311,7 @@ abstract class DiktatSmokeTestBase : FixTestBase("test/smoke/src/main/kotlin",
 
     companion object {
         const val DEFAULT_CONFIG_PATH = "../diktat-analysis.yml"
-        private const val TEST_TIMEOUT_SECONDS = 20L
+        private const val TEST_TIMEOUT_SECONDS = 25L
         val unfixedLintErrors: MutableList<LintError> = mutableListOf()
 
         // by default using same yml config as for diktat code style check, but allow to override