flake: TestGracefulBuildCancel

[balopat]

Sometimes OSX travis tests fail with TestGracefulBuildCancel failures. We should eliminate these flakes.

[balopat]

Happened again,

/pkg/skaffold/build/misc/TestGracefulBuildCancel/terminate_gracefully_and_exit_0
    --- FAIL: TestGracefulBuildCancel/terminate_gracefully_and_exit_0 (0.24s)
        graceful_test.go:64: unexpected error: signal: killed
/pkg/skaffold/build/misc/TestGracefulBuildCancel
--- FAIL: TestGracefulBuildCancel (0.52s)

[dgageot]

I've seen this test fail very often those days.