minor adjustments to test_pr

[minrk]

• scrub build between runs (sometimes required for 2to3 to behave properly, possibly only on OS X)
• split git pull into git pull --no-commit and git commit -m foo, to prevent invoking editor for the commit message (If there is a single command like git pull -m, I am not aware of it).
• time run_tests (just because)

[takluyver]

Test results for commit 5c3a190 merged into master
Platform: linux2

• python2.7: OK (libraries not available: matplotlib pymongo wx wx.aui)
• python3.2: OK (libraries not available: pymongo wx wx.aui)

Not available for testing:

[takluyver]

Seems to be OK. I'm not sure why git pull asks you for a commit message but not me, but that's life. Thanks for this, I'll merge it now.