xdgmime: find test programs via bare name first, and wire up subprojects

Instead of / in addition to hardcoding the path to the program name via
a Meson option try to find just the name, first.

This means that there are two chances to find it, once via PATH or
program overrides and once via specified location.

It is also possible to use the `dirs: ` kwarg to find_program, but that
requires raising the minimum version of Meson (to 0.53.0). Specifying
two candidate names works quite well too, and is backwards-compatible.

Take advantage of this by:

- adding an option to make the tests be required, so that Meson tries
  extra hard to find the programs by adding new subprojects into the
  build
- adding a wrap file that automatically clones the xdgmime repository
  and builds it if the xdgmime programs cannot be found, then overrides
  the found programs using the built versions
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