Avoid clash between source field and header field in CsvProcessorTests

This change fixes flakiness in `CsvProcessorTests` where source field
can be the same as one of the headers used by tests which messes up
asserts when we check that field is not present after processor run.

Closes elastic#50209

Author: probakowski

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

@@ -223,8 +223,9 @@ private IngestDocument processDocument(String[] headers, String csv) throws Exce
     private IngestDocument processDocument(String[] headers, String csv, boolean trim) throws Exception {
         IngestDocument ingestDocument = RandomDocumentPicks.randomIngestDocument(random());
         Arrays.stream(headers).filter(ingestDocument::hasField).forEach(ingestDocument::removeField);
+        String fieldName = randomAlphaOfLength(11);
+        ingestDocument.setFieldValue(fieldName, csv);
 
-        String fieldName = RandomDocumentPicks.addRandomField(random(), ingestDocument, csv);
         char quoteChar = quote.isEmpty() ? '"' : quote.charAt(0);
         CsvProcessor processor = new CsvProcessor(randomAlphaOfLength(5), fieldName, headers, trim, separator, quoteChar, false);