Unit tests flake in TestInfeasibleRetry

[jsafrane]

Unit tests flake often (~5% of runs) with:

--- FAIL: TestInfeasibleRetry (0.30s)
    --- FAIL: TestInfeasibleRetry/Should_NOT_retry_infeasible_error_normally (0.10s)
        controller_test.go:324: for Should NOT retry infeasible error normally, unexpected second syncPVC error: rpc error: code = InvalidArgument desc = fake infeasible error
        controller_test.go:337: for Should NOT retry infeasible error normally: expected 1 csi modify calls, but got 2

Easily reproducible with:

cd pkg/modifycontroller
go test -c
stress ./modifycontroller.test

Tested with today's master, cba05cb