Avoid http client race in test case.

Avoid http client race in test case.

If we don't wait for Put() to return before returning from the test
case, CloseIdleConnections can cause Put() to fail, and the resulting
call to t.Error() panics the testing package.

"errType *url.Error, err Put
http://fakeaccountname.blob.127.0.0.1:42861/fakecontainername/e4d909c290d0fb1ca068ffaddf22cbd0:
net/http: HTTP/1.x transport connection broken: http:
CloseIdleConnections called"

"panic: Fail in goroutine after TestAzureBlobVolumeCreateBlobRace has completed"

No issue #

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tclegg@veritasgenetics.com>