Run tests under Meson again

• tests: Generate gtkdocentities.xml during Meson build

  Otherwise tests will fail when we can't load external entities from it.

• tests: Copy -sections.txt to build directory where needed

  Two tests rely on symbols in and being ignored. The Autotools build automatically copies -sections.txt into the build directory where gtk-doc will find it, but the Meson build does not, resulting in gtk-doc generating a new -sections.txt that does not know which symbols to ignore, and complaining that the private symbols are now undocumented.

• Revert "Ignore failing test swith meson for now"

  This reverts commit 1f0a7303.

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/cc @xclaesse

Fans of Meson should also consider !48 (merged).