test: properly gate a zstd section

This part of the test can't run if zstd is not available. This was caught by
--pure test (who don't support zstd).

tests/test-repo-compengines.t

@@ -139,6 +139,8 b' Test error cases'
   abort: invalid value for `storage.revlog.zlib.level` config: 42
   [255]
 
+#if zstd
+
 checking zstd options
 =====================
 
@@ -194,3 +196,4 b' Test error cases'
   abort: invalid value for `storage.revlog.zstd.level` config: 42
   [255]
 
+#endif