TST: fix executable bit test

[rgommers]

Note that Meson does have the correct install path in intro-install_plan.json, containing {bindir}.
This gets translated into script/ somewhere, at least for a conda-installed Python on Arch Linux. This code was added in gh-102, and it's not quite clear why this line was commented out.

For now, let's just make sure the test is not failing.

[rgommers]

This all looks good except for the Mypy failure, which is fixed in gh-149. Somehow even the Gentoo job turned green.

[rgommers]

With this change, the failing test in gh-143 also passes. So I'd say we merge this, and deal with bin/ vs. script/ later. That really isn't important, we need to fix the issues that are affecting SciPy and Pandas urgently.

[rgommers]

This is all happy, the one failure is a weird job runner issue where the log isn't visible and it can't even be re-run. Fixes the local failures I'm seeing.

[rgommers]

Ah, the issue with bin vs script is discussed in gh-115. We should be good here. Let's merge this, and leave gh-115 open for the follow-up to ensure everything is 100% correct (it will be with Meson master it looks like, not yet with 0.63.2). I will comment there, and just update the code comment here to point at that issue.