Improve our test isolation

[jameinel]

We make heavy use of PatchValue in our test suite. But the existing code actually had several failure modes.

1. SetUpSuite(){PatchValue()} would actually add the Cleanup call to the first TearDownTest, which means that it only held true for one test.
2. Test suites that would call PatchValue, but not properly call TearDown, would leave values patched permanently. This includes cases where a test was written without a "pointer" receiver. (func (f Foo) TestBar(c *gc.C)). That actually copies the stack of things to be cleaned up, and mutates the copy, and when the TestBar function returns, the stack resets to its original value.
3. Test suites would call PatchValue, and then call CleanupSuite.SetUpTest(). What they didn't realize is that SetUpTest() resets the list of things to be cleaned up to 'nil'. Which meant we would never run any of the cleanups.

I submitted a patch to github.com/juju/testing that changed PatchValue to be safer, and refuse to patch when it looks like we weren't doing what you want. This change against Juju Core brings in the updated testing package, and fixes all of the places that failed because they weren't actually setting themselves up correctly.

The safety checks also mean that you have to call CleanupSuite.SetUpTest() before you call PatchValue for that test. Which required moving some PatchValues around. Sometimes moving things from SetUpTest to SetUpSuite if they needed to be patched before Base.SetUpTest() is called. (we have a few places that need to inject Clock or something like that, into the service that is started in SetUpTest.)

Everything passes on my Trusty box, but I can't be sure it will pass on all platforms.

(Review request: http://reviews.vapour.ws/r/4240/)

[jameinel]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/6962

[jameinel]

$$merge$$ looks like someone landed a new test suite that didn't call BaseSuite.

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju