[Telemetry] Fix potential flake in testPagesetRepeat.
[Telemetry] Fix potential flake in testPagesetRepeat.

A previous patch of mine (which seems unrelated) triggered flake in os.remove()
complaining that the file was still open. It seems like there could be a race
with closing the file here. The with-block should make it deterministic.

BUG=223572

Review URL: https://codereview.chromium.org/256413002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@265825 0039d316-1c4b-4281-b951-d872f2087c98

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