test: hashmap - increase number of entries for crippled hash test

The purpose of testing with the crippled hash function is to cover
the otherwise very unlikely codepath in bucket_calculate_dib() where
it has to fall back to recomputing the hash value.

This unlikely path was not covered by test-hashmap anymore after
57217c8f "test: hashmap - cripple the hash function by truncating the
input rather than the output".

Restore the test coverage by increasing the number of entries in the test.
The number was determined empirically by checking with lcov.

src/test/test-hashmap-plain.c

@@ -710,7 +710,7 @@ static void test_hashmap_many(void) {
                 unsigned n_entries;
         } tests[] = {
                 { .ops = NULL,                  .n_entries = 1 << 20 },
-                { .ops = &crippled_hashmap_ops, .n_entries = 1 << 11 },
+                { .ops = &crippled_hashmap_ops, .n_entries = 1 << 14 },
         };