test-from-egg and test-from-prefixdir are not testing the right code

[davidsarah]

The test-from-egg and test-from-prefixdir buildbot steps are supposed to be testing the same code that was built, but they may actually test a previous version that is installed in site-packages.

For example,

• ​http://tahoe-lafs.org/buildbot/builders/Kyle%20OpenBSD-4.6%20amd64/builds/316/steps/test-from-egg/logs/stdio
• ​http://tahoe-lafs.org/buildbot/builders/Kyle%20OpenBSD-4.6%20amd64/builds/316/steps/test-from-prefixdir/logs/stdio

are supposed to be testing 1.7.1-r4600 on the ticket1074 branch, but are actually testing the 1.7.1 release.

[davidsarah]

I wrote in the original description:

To detect this problem, we could have the commands that run these steps set an environment variable, say REQUIRED_TAHOE_VERSION, and then fail the tests if that is set and doesn't match allmydata.__version__.

Actually this wouldn't be enough. We're not just supposed to be testing the right version, we're supposed to be testing a specific egg or installation, so that we know that it is usable (not missing any needed files, for example). So we need to:

1. Fail if the code we are testing is not loaded from the expected egg or prefix directory.
2. Fail if the code in that directory is not the right Tahoe version. (Or, equivalently given that check 1 passes: fail if the code we are testing is not the right Tahoe version.)
3. Run the tests in such a way that the preceding checks don't fail.

We should also consider checking constraints on the versions of our dependencies, but I think that's a separate ticket.