update sample tests, fix Java tests (#20300)

* replace removed forkMode

* remove junit runner where it's not needed

* update samples without skipping test files, but skip files named "FILES"

* revert overwriting custom tests, add custom java tests to list

* add one sample to CircleCI, fix various Java tests

samples/server/petstore/kotlin-springboot-bigdecimal-default/src/test/kotlin/org/openapitools/api/TestApiTest.kt

@@ -16,8 +16,9 @@ class TestApiTest {
      */
     @Test
     fun testPostTest() {
-        // val apa: Apa = TODO()
-        // val response: ResponseEntity<Unit> = api.testPost(apa)
+        val apa: Apa = TODO()
+        val response: ResponseEntity<Unit> = api.testPost(apa)
+
         // TODO: test validations
     }
 }