Add "tests" fields to test qlpacks

This will allow `codeql resolve tests --ignore-dubious-cases`
(and thus the VSCode extension) to recognize all `.ql` files in those
packs as test cases, even if they don't have accompanying `.expected`
files.

CLI versions prior to 2.1.0 will choke on this, but it's almost 10
months since that came out.

cpp/ql/test/qlpack.yml

@@ -2,3 +2,4 @@ name: codeql-cpp-tests
 version: 0.0.0
 libraryPathDependencies: codeql-cpp
 extractor: cpp
+tests: .