[SPARK-29918][SQL][FOLLOWUP][TEST] Fix arrayOffset in RecordBinaryComparatorSuite

### What changes were proposed in this pull request?

As mentioned in #26548 (review), some test cases in `RecordBinaryComparatorSuite` use a fixed arrayOffset when writing to long arrays, this  could lead to weird stuff including crashing with a SIGSEGV.

This PR fix the problem by computing the arrayOffset based on `Platform.LONG_ARRAY_OFFSET`.

### How was this patch tested?
Tested locally. Previously, when we try to add `System.gc()` between write into long array and compare by RecordBinaryComparator, there is a chance to hit JVM crash with SIGSEGV like:
```
#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007efc66970bcb, pid=11831, tid=0x00007efc0f9f9700
#
# JRE version: OpenJDK Runtime Environment (8.0_222-b10) (build 1.8.0_222-8u222-b10-1ubuntu1~16.04.1-b10)
# Java VM: OpenJDK 64-Bit Server VM (25.222-b10 mixed mode linux-amd64 compressed oops)
# Problematic frame:
# V  [libjvm.so+0x5fbbcb]
#
# Core dump written. Default location: /home/jenkins/workspace/sql/core/core or core.11831
#
# An error report file with more information is saved as:
# /home/jenkins/workspace/sql/core/hs_err_pid11831.log
#
# If you would like to submit a bug report, please visit:
#   http://bugreport.java.com/bugreport/crash.jsp
#
```
After the fix those test cases didn't crash the JVM anymore.

Closes #26939 from jiangxb1987/rbc.

Authored-by: Xingbo Jiang <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: jiangxb1987

sql/core/src/test/java/test/org/apache/spark/sql/execution/sort/RecordBinaryComparatorSuite.java

@@ -325,7 +325,7 @@ public void testBinaryComparatorWhenOnlyTheLastColumnDiffers() throws Exception
 
   @Test
   public void testCompareLongsAsLittleEndian() {
-    long arrayOffset = 12;
+    long arrayOffset = Platform.LONG_ARRAY_OFFSET + 4;
 
     long[] arr1 = new long[2];
     Platform.putLong(arr1, arrayOffset, 0x0100000000000000L);
@@ -347,7 +347,7 @@ public void testCompareLongsAsLittleEndian() {
 
   @Test
   public void testCompareLongsAsUnsigned() {
-    long arrayOffset = 12;
+    long arrayOffset = Platform.LONG_ARRAY_OFFSET + 4;
 
     long[] arr1 = new long[2];
     Platform.putLong(arr1, arrayOffset + 4, 0xa000000000000000L);