testing/minio: upgrade to 0.20190918

Significant changes:
- Now that we are on go 1.13, enabling go111modules isn't needed
- With full go111modules support GOPATH is mostly there for caching
- Some new logic is failing cmd tests (DiskUsage function)
1 job for !69 with minio in 19 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Lint
failed #733
x86_64 docker-alpine allowed to fail
lint

00:00:19

 
Name Stage Failure
failed
lint Lint
No job log