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.

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__.