rustpkg tests shouldn't compare modification times

[catamorphism]

The tests shouldn't compare the results of mtimes from util::datestamp. I fixed one of the tests, but others are failing now too, and then my fix for it failed on the linux bot. I'm investigating.

[catamorphism]

I'm working on this, but the scratch directory on the linux bot that I was working out of mysteriously decided to rebuild LLVM again, and I have to leave for the day, so I'll get to it tomorrow.

[catamorphism]

I'm about to check in a fix that will unignore all the tests except for test_rebuild_when_needed. That one still fails on Linux for some reason. When it can be un-ignored on Linux, this bug can be closed.

[catamorphism]

I changed the tests so that they set the output files to read-only in order to check for unwanted rebuilding. The problem is that on Linux, the linker will silently overwrite a read-only file. I thought I had a fix for this (checking in rustc, before invoking the linker, that the output file doesn't exist as a read-only file already) but that failed on the try bots, too, and since I don't have VPN access, I can't log into the bot to experiment right now.