tests: wait for index refresh before getting document count (#70)

i've several times observed flaky tests which seem to boil down to the
fact that the index hadn't been refreshed yet before we queried the
count.

this happens because writing a document does not make it immediately
available for querying, unless you specify the `refresh=true` parameter
in the request.

thus there were now two ways of fixing this:
* add `refresh=true` in the migrations
* execute an index refresh before querying the count

as the test files might be used as templates by other users i decided to
go for the second approach, as this way the executions run faster (not
relevant if you add a single document, but very relevant when you do
larger migrations).
as we currently only rely on index creation (no refresh needed) and
document count (the modified function) there's only a single place which
had to be extended with this. if we ever add tests which use other
metrics relying on the content of an index we'll have to add the same
there.

src/test/java/liquibase/ext/opensearch/AbstractOpenSearchLiquibaseIT.java

@@ -70,6 +70,7 @@ protected boolean indexExists(final String indexName) throws Exception {
     }
 
     protected long getDocumentCount(final String indexName, final Query query) throws Exception {
+        this.getOpenSearchClient().indices().refresh(r -> r.index(indexName));
         final var request = new CountRequest.Builder()
                 .index(indexName)
                 .query(query)