build: use "meson test" as the test harness

"meson test" starting with version 0.57 is just as capable and easy to
use as QEMU's own TAP driver.  All existing options for "make check"
work.  The only required code change involves how to mark "slow" tests;
they need to belong to an additional "slow" suite.

The rules for .tap output are replaced by JUnit XML; GitLab is able
to parse that output and present it in the CI pipeline report.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>