HSEARCH-2762 Lower the JVM heap size of Elasticsearch 5+ instances sp…

…awned in tests

We could do it for ES2 too, but that would require messing with
environment variables, and doing so from Maven doesn't seem easy. So we
won't bother.

sharedtestresources/src/main/elasticsearch-maven-plugin/5.0/configuration/jvm.options

@@ -19,8 +19,11 @@
 # Xms represents the initial size of total heap space
 # Xmx represents the maximum size of total heap space
 
--Xms2g
--Xmx2g
+# For Hibernate Search, we don't need as much as the default 2g
+# Let's keep it low, so that we'll be able to run tests
+# on memory-constrained CI slaves
+-Xms256m
+-Xmx256m
 
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