[SPARK-23892][TEST] Improve converge and fix lint error in UTF8String-related tests

## What changes were proposed in this pull request?

This PR improves test coverage in `UTF8StringSuite` and code efficiency in `UTF8StringPropertyCheckSuite`.

This PR also fixes lint-java issue in `UTF8StringSuite` reported at [here](apache#20995 (comment))

```[ERROR] src/test/java/org/apache/spark/unsafe/types/UTF8StringSuite.java:[28,8] (imports) UnusedImports: Unused import - org.apache.spark.unsafe.Platform.```

## How was this patch tested?

Existing UT

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes apache#21000 from kiszk/SPARK-23892.

Author: kiszk

common/unsafe/src/test/java/org/apache/spark/unsafe/types/UTF8StringSuite.java

@@ -25,7 +25,6 @@
 import java.util.*;
 
 import com.google.common.collect.ImmutableMap;
-import org.apache.spark.unsafe.Platform;
 import org.apache.spark.unsafe.memory.ByteArrayMemoryBlock;
 import org.apache.spark.unsafe.memory.OnHeapMemoryBlock;
 import org.junit.Test;
@@ -53,8 +52,8 @@ private static void checkBasic(String str, int len) {
 
     assertTrue(s1.contains(s2));
     assertTrue(s2.contains(s1));
-    assertTrue(s1.startsWith(s1));
-    assertTrue(s1.endsWith(s1));
+    assertTrue(s1.startsWith(s2));
+    assertTrue(s1.endsWith(s2));
   }
 
   @Test

common/unsafe/src/test/scala/org/apache/spark/unsafe/types/UTF8StringPropertyCheckSuite.scala

@@ -164,7 +164,7 @@ class UTF8StringPropertyCheckSuite extends FunSuite with GeneratorDrivenProperty
     def padding(origin: String, pad: String, length: Int, isLPad: Boolean): String = {
       if (length <= 0) return ""
       if (length <= origin.length) {
-        if (length <= 0) "" else origin.substring(0, length)
+        origin.substring(0, length)
       } else {
         if (pad.length == 0) return origin
         val toPad = length - origin.length