Ensure that compile-flags arguments are the last in UI tests
Before this PR, compiletest would add `-L path/to/aux` at the end of the rustc flags, even after the custom ones set with the compile-flags header comment. This made it impossible to check how rustc would behave when a flag requiring an argument was passed without the argument, because the argument would become `-L`.
This PR fixes that by adding the `-L path/to/aux` before the arguments defined in compile-flags, at least for UI tests. Other test suites might either be fixed as well by this change, or still present the old behavior (`-L` is now always passed before, but other tests suites might add additional flags after the custom ones).
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