xbps-src: XBPS_CHECK_PKGS with cross-builds

XBPS_CHECK_PKGS is a global "do we test packages" check, set with the -Q option or in etc/conf.

During cross builds this variable is still set true despite checkdepends not being installed and do_check not being used.

A lot of templates just check [ "$XBPS_CHECK_PKGS" ] which works in most cases, but not in cases where its used to explicitly disable tests because the build system might depend on something in checkdepends if tests are enabled:

if [ -z "$XBPS_CHECK_PKGS" ]; then
    configure_args+=" -Dtests=false"
fi

xbps-src itself usually does a check like [ "$XBPS_CHECK_PKGS" ] && [ -z "$XBPS_CROSS_BUILD" ].

I think we should either change XBPS_CHECK_PKGS to indicate whether checks are going to be run or add a new variable for that.

Start with xbps-src handling of XBPS_CHECK_PKGS and XBPS_CROSS_BUILD, then inspect the default in etc/conf and templates that use the variable to disable tests. Determine and document the intended cross-build semantics, then verify that package checks and checkdepends-dependent test configuration behave consistently.