HDFS-15731. Reduce threadCount for unit tests to reduce the memory usage (#2556)

Reviewed-by: Ayush Saxena <ayushsaxena@apache.org>
(cherry picked from commit 9cf408d6baf2fa9071729cac57516d366e928a9f)

hadoop-project/pom.xml

@@ -40,7 +40,7 @@
     <test.exclude.pattern>_</test.exclude.pattern>
 
     <!-- number of threads/forks to use when running tests in parallel, see parallel-tests profile -->
-    <testsThreadCount>4</testsThreadCount>
+    <testsThreadCount>2</testsThreadCount>
 
     <!-- These 2 versions are defined here because they are used -->
     <!-- JDIFF generation from embedded ant in the antrun plugin -->