Fix hang in test for "too many fields" dep. check (elastic#42909)

This commit fixes a rare case where the method to randomly generate a
very large mapping could enter an infinite loop.

Author: gwbrown

x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecksTests.java

@@ -370,7 +370,7 @@ private static void addRandomField(List<String> existingFieldNames, final int fi
                 mappingBuilder.startObject("properties");
                 {
                     int subfields = randomIntBetween(1, 10);
-                    while (existingFieldNames.size() < subfields) {
+                    while (existingFieldNames.size() < subfields && fieldCount.get() <= fieldLimit) {
                         addRandomField(existingFieldNames, fieldLimit, mappingBuilder, fieldCount);
                     }
                 }