Grok processor ignore case test

Added test demonstrating that grok using ignore case works, since this
does a minimal test that the `joni` and `jcodings` libraries are
compatible.

Forward-port of test from elastic#43334

Author: henningandersen

modules/ingest-common/src/test/java/org/elasticsearch/ingest/common/GrokProcessorTests.java

@@ -45,6 +45,16 @@ public void testMatch() throws Exception {
         assertThat(doc.getFieldValue("one", String.class), equalTo("1"));
     }
 
+    public void testIgnoreCase() throws Exception {
+        String fieldName = RandomDocumentPicks.randomFieldName(random());
+        IngestDocument doc = RandomDocumentPicks.randomIngestDocument(random(), new HashMap<>());
+        doc.setFieldValue(fieldName, "A");
+        GrokProcessor processor = new GrokProcessor(randomAlphaOfLength(10), Collections.emptyMap(),
+            Collections.singletonList("(?<a>(?i)A)"), fieldName, false, false, MatcherWatchdog.noop());
+        processor.execute(doc);
+        assertThat(doc.getFieldValue("a", String.class), equalTo("A"));
+    }
+
     public void testNoMatch() {
         String fieldName = RandomDocumentPicks.randomFieldName(random());
         IngestDocument doc = RandomDocumentPicks.randomIngestDocument(random(), new HashMap<>());