Add info about filtering of legacy tests

Author: tsayukov

README.md

@@ -138,4 +138,14 @@ Running integration tests only:
 
 ```bash
 ctest -j$(nproc) --preset release-integration
-```
+```
+
+Note that some tests use a legacy test library instead of GoogleTest, see `./<test_target> --help` for details. If you need to run only certain test cases, here is an alternative for `--gtest_filter` option:
+
+```bash
+cat <<EOF | ./<test_target> --filter-file /dev/fd/0
+-ExcludedTestCase
++IncludedTestCase
++IncludedTestCase::TestName
+EOF
+```