tests: avoid unnecessary rebuilds
tests: avoid unnecessary rebuilds

Improve dependency tracking on vapi_test directory so that vapi_c_test
rebuild doesn't force vapi_cpp_test rebuild and vice versa.

Type: improvement
Signed-off-by: Klement Sekera <b8003f27414ca85c37624503b7b4e8f8c829bb4d@cisco.com>
Change-Id: I00094c6374981333858ddbe6ae934cfda3b33a36

chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp