meson: add option to disable tests

[bgilbert]

The check dependency is only needed for building tests. Allow avoiding it by disabling tests. However, require an explicit configuration setting to do so, and continue making check mandatory when tests are enabled, so users/packagers can't accidentally skip running tests when they intend to do so.

This is a partial reversal of a change I made in #23, which assumed that the wrapdb fallback made it okay to require check. That's fine for end users, but requires openslide-winbuild (which doesn't use wraps from subprojects) to carry a check wrap even though it has no intention of running unit tests. This PR splits the difference by making the disablement entirely manual, rather than basing it on the result of feature detection.

[jcupitt]

I merged this to main on my libdicom fork so I can test with winbuild.