Replace Singular example with GAP in _get_shared_lib_path()

[orlitzky]

We still have a doctest for _get_shared_lib_path() that ensures it can find libSingular. However,

1. the premise behind _get_shared_lib_path() does not make sense; you can't just guess where a library comes from
2. as a result of (1), it's failing to find libSingular in some cases; but thankfully
3. we no longer care if it can find libSingular, because the full path to libSingular is contained in LIBSINGULAR_PATH

Instead we should have it look for libgap, because gap is still installed from an SPKG, and we can ensure that it's installed in just the right way for _get_shared_lib_path() to find it. Moreover, we actually call the function with "gap" as an argument, so there's that.

Reported in #29024 comment:168

Component: misc

Author: Michael Orlitzky

Branch/Commit: 17e308f

Reviewer: Gonzalo Tornaría

Issue created by migration from https://trac.sagemath.org/ticket/32880

[orlitzky]

Author: Michael Orlitzky

[orlitzky]

New commits:

17e308f

Trac #32880: use GAP instead of Singular to test _get_shared_lib_path().

[orlitzky]

Branch: u/mjo/ticket/32880

[orlitzky]

Commit: 17e308f

[tornaria]

comment:3

Looks good to me. I took the patch for the sagemath package for void linux (wip) which uses system singular, and the failing doctest now passes.

See ​void-linux/void-packages#34030

[tornaria]

Reviewer: Gonzalo Tornaría

[vbraun]

Changed branch from u/mjo/ticket/32880 to 17e308f