Add ability for all_tests.sh to run based on prefix

Adds ability to run all tests starting with a specific prefix

Now, we can run things like:
all_tests.sh --prefix xfrm
all_tests.sh -p xfrm

and it will run all test files that start with xfrm

Note that only the last prefix provided will be used.

Test: script running tests correctly, along with flake testing.
Failures are correctly reflected in the exit code. Ran tests with and
without prefix, on flaky tests, and with and without environment
variables.
Change-Id: Ib47d107e52916a8033676b1b8ad99d924bb3cd73