Testing: Simplify UploadContent on linux

[martijnvans]

Description

This PR is effectively undoing cl/378416757, because RunRemotelyStdin has been fixed (#1636) to handle large stdin values.

This PR also adds some tests for UploadContent, including a test that failed prior to #1636 (but passes now).

This PR also changes the contract of UploadContent to no longer automatically create enclosing directories; instead the burden of doing that has been moved to the caller. Only one callsite needed to be fixed, though that required implementing a new function makeParentDirectory in ops_agent_test.go.

The net effect of this PR is that:

• UploadContent doesn't depend on InstallGsutilIfNeeded anymore
• UploadContent doesn't use GCS or TRANSFERS_BUCKET on linux anymore
• InstallGsutilIfNeeded can move out of the core gce_testing.go library into the agents library, the only place that it is still used.

Related issue

cleanup

How has this been tested?

automated tests passed, plus I ran gce_testing_test.go manually, also passing.

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.