Brightness curve test reports data from previous test

Expected Output:
```
u
758108 11
758706 11
759308 11
.
.
.
end
```

Actual Output:
```
1737914 11
1739008 11
u
758108 11
758706 11
759308 11
.
.
.
end
```

A few data points from the end of the previous test are being reported in the next test. This bug does not appear the first time you run the brightness curve test, but on every test after that exactly 2 erroneous lines are printed before the beginning of the current test (marked with `u`). This bug does not appear on ChromeOS with the tcp bridge setup, and it does not appear on Android with the usb hub/ethernet setup.

Might be related: when running the brightness curve test, and then running Diagnostics -> Reconnect, an error is printed:
```Unable to communicate with WALT: Unexpected response from WALT. Expected "v", got ""```