Send measurement if the test ended before UpdateInterval

[cristinaleonr]

This PR adds functionality to send the measurement through the channel if one is received before params.UpdateInterval. It used to not be likely that a test would terminate before said interval, but this is now possible with the early-exit changes to the server.

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This change is 

[stephen-soltesz]

Reviewable status: 0 of 1 approvals obtained (waiting on @cristinaleonr)

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internal/download/download.go line 44 at r1 (raw file):

			// If the test finished before `params.UpdateInterval` and at least one message
			// has been received, send the measurement data through the channel before exiting.
			if elapsed <= params.UpdateInterval && total > 0 {

nit: there is a race here based on update interval and elapsed time. I can imagine scenarios where the elapsed time below is less than UpdateInterval but by the time we get here it is greater than.

What if we set a boolean below instead? And, report a measurement here only if that boolean is unset?

[cristinaleonr]

Reviewable status: 0 of 1 approvals obtained

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internal/download/download.go line 44 at r1 (raw file):

Previously, stephen-soltesz (Stephen Soltesz) wrote…

nit: there is a race here based on update interval and elapsed time. I can imagine scenarios where the elapsed time below is less than UpdateInterval but by the time we get here it is greater than.

What if we set a boolean below instead? And, report a measurement here only if that boolean is unset?

Done.

[stephen-soltesz]

Reviewable status: complete! 1 of 1 approvals obtained