Cover patch_linux.go by unit tests

[iliatsuprik]

This PR contains unit tests and minor changes in code for github.com/GoogleCloudPlatform/osconfig/agentendpoint/patch_linux.go.
Initial coverage: 23.4%, current coverage: 100%

The following variables were moved to be global (needed to mock them):

runAptGetUpgrade = ospatch.RunAptGetUpgrade
runYumUpdate     = ospatch.RunYumUpdate
runZypperPatch   = ospatch.RunZypperPatch
retryPeriod      = 3 * time.Minute

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: iliatsuprik
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[google-oss-prow]

Hi @iliatsuprik. Thanks for your PR.

I'm waiting for a GoogleCloudPlatform member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[burov]

/gcbrun