8042983: test/java/math/BigInteger/ExtremeShiftingTests.java needs too much heap

Use @requires tag to suppress the test on limited memory systems.

Reviewed-by: darcy

jdk/test/java/math/BigInteger/ExtremeShiftingTests.java

@@ -25,6 +25,7 @@
  * @test
  * @bug 6371401
  * @summary Tests of shiftLeft and shiftRight on Integer.MIN_VALUE
+ * @requires os.maxMemory >= 1g
  * @run main/othervm -Xmx512m ExtremeShiftingTests
  * @author Joseph D. Darcy
  */