Fix slow tests caused by retry backoff

[armsnyder]

I disabled retry backoffs in tests to reduce the overall test run time from 25 seconds to under 1 second.

Before:

% ~/go/bin/gotestsum --jsonfile out.json
✓  . (25.685s)
% ~/go/bin/gotestsum tool slowest --jsonfile out.json
github.com/xanzy/go-gitlab TestInstanceClustersService_AddCluster_StatusInternalServerError 12.08s
github.com/xanzy/go-gitlab TestInstanceVariablesService_StatusInternalServerError 11.73s
github.com/xanzy/go-gitlab TestUploadFile_Retry 710ms
github.com/xanzy/go-gitlab TestUploadAvatar_Retry 710ms

After:

% ~/go/bin/gotestsum --jsonfile out.json
✓  . (425ms)
% ~/go/bin/gotestsum tool slowest --jsonfile out.json

[svanharmelen]

I've taken another look at the involved tests and guess you are right... So I'll take it... 👍🏻