Update testing-sources.md for docs on running all source tests

The documentation advises on running the `dbt test --select source:*` command for running tests on all sources. However, this syntax (with the asterisk), is not something the CLI is too happy with. Double quoting the selector resolves the error.

website/docs/faqs/Tests/testing-sources.md

@@ -9,7 +9,7 @@ id: testing-sources
 To run tests on all sources, use the following command:
 
 ```shell
-$ dbt test --select source:*
+$ dbt test --select "source:*"
 ```
 
 (You can also use the `-s` shorthand here instead of `--select`)