[llvm] [lit] Fix `progress-bar.py` flaky test (PR #193741)

[Charles Zablit via llvm-commits]

charles-zablit wrote:

> Do you think there's some "minimal" part of progress bar that's guaranteed to appear at every step, so we could get away with checking whether each line starts with something?

I agree that we should still test the progress bar's progress somehow.

You mentioned that Filecheck was not always matching the expected line. Would it make more sense to write the test in Python by hand and check the output with Python directly rather than relying on Filecheck?

https://github.com/llvm/llvm-project/pull/193741