[SPARK-23882][CORE] UTF8StringSuite.writeToOutputStreamUnderflow() is not expected to be supported

## What changes were proposed in this pull request?

This PR excludes an existing UT [`writeToOutputStreamUnderflow()`](https://github.com/apache/spark/blob/master/common/unsafe/src/test/java/org/apache/spark/unsafe/types/UTF8StringSuite.java#L519-L532) in `UTF8StringSuite`.

As discussed [here](apache#19222 (comment)), the behavior of this test looks surprising. This test seems to access metadata area of the JVM object where is reserved by `Platform.BYTE_ARRAY_OFFSET`.

This test is introduced thru apache#16089 by NathanHowell. More specifically, [the commit](apache@27c102d) `Improve test coverage of UTFString.write` introduced this UT. However, I cannot find any discussion about this UT.

I think that it would be good to exclude this UT.

```java
  public void writeToOutputStreamUnderflow() throws IOException {
    // offset underflow is apparently supported?
    final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
    final byte[] test = "01234567".getBytes(StandardCharsets.UTF_8);

    for (int i = 1; i <= Platform.BYTE_ARRAY_OFFSET; ++i) {
      new UTF8String(
        new ByteArrayMemoryBlock(test, Platform.BYTE_ARRAY_OFFSET - i, test.length + i))
          .writeTo(outputStream);
      final ByteBuffer buffer = ByteBuffer.wrap(outputStream.toByteArray(), i, test.length);
      assertEquals("01234567", StandardCharsets.UTF_8.decode(buffer).toString());
      outputStream.reset();
    }
  }
```

## How was this patch tested?

Existing UTs

Author: Kazuaki Ishizaki <[email protected]>

Closes apache#20995 from kiszk/SPARK-23882.

Author: kiszk

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

@@ -515,22 +515,6 @@ public void soundex() {
     assertEquals(fromString("世界千世").soundex(), fromString("世界千世"));
   }
 
-  @Test
-  public void writeToOutputStreamUnderflow() throws IOException {
-    // offset underflow is apparently supported?
-    final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
-    final byte[] test = "01234567".getBytes(StandardCharsets.UTF_8);
-
-    for (int i = 1; i <= Platform.BYTE_ARRAY_OFFSET; ++i) {
-      new UTF8String(
-        new ByteArrayMemoryBlock(test, Platform.BYTE_ARRAY_OFFSET - i, test.length + i))
-          .writeTo(outputStream);
-      final ByteBuffer buffer = ByteBuffer.wrap(outputStream.toByteArray(), i, test.length);
-      assertEquals("01234567", StandardCharsets.UTF_8.decode(buffer).toString());
-      outputStream.reset();
-    }
-  }
-
   @Test
   public void writeToOutputStreamSlice() throws IOException {
     final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();