Create static gather_tests utility method
Update run-ci to use static method
Replace benchmarker's gather_tests with static method
Fix installation bug, replace gather_tests with static call

Installer was conflating test directory with test name, which meant
that tests like aspnet-mono were not triggering a call to aspnet/install.sh
Merge pull request #969 from hamiltont/refactor_gather_tests

Refactor gather tests into static call