Improve test runner documentation

Didn't explain some useful features explained by the command help

Author: jasalt

content/documentation/testing.md

@@ -75,7 +75,24 @@ Test can be defined by using the `deftest` macro. This macro is like a function
 
 Tests can be run using the `./vendor/bin/phel test` command. Tests are looked up recursively in all directories set by [setTestDirs](/documentation/configuration/#testdirs) configuration option which defaults to `tests/`.
 
-If you want to run the test manually, the `run-tests` function can be used. As arguments, it takes a map of options (that can be empty) and one or more namespaces that should be tested.
+Pass filenames as arguments to the `phel test` command to run tests in specified files only:
+
+```bash
+./vendor/bin/phel test tests/main.phel tests/utils.phel
+```
+
+To filter tests that should run by name, `--filter` command line argument can be used:
+
+```bash
+./vendor/bin/phel test tests/utils.phel --filter my-test-function
+```
+
+Test report can be set to more verbose TestDox format showing individual test names with `--testdox` flag. Output can also be suppressed with `--quiet` flag to only include errors or silenced fully with `--silent` flag.
+
+See more options available by running `./vendor/bin/phel test --help`.
+
+
+If you want to run tests from Phel code, the `run-tests` function can be used. As arguments, it takes a map of options (that can be empty) and one or more namespaces that should be tested.
 
 ```phel
 (run-tests {} 'my\ns\a 'my\ns\b)