tests: tickless_concept: disable for coverage

This test is already flaky, but becomes even flakier when
coverage is enabled.

Disable until we put a stake through the QEMU timing issues
being worked on in zephyrproject-rtos#14173.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

tests/kernel/tickless/tickless_concept/testcase.yaml

@@ -1,4 +1,7 @@
 tests:
   kernel.tickless:
     arch_exclude: riscv32 nios2
+    # FIXME: This test fails sporadically on all QEMU platforms, but fails
+    # consistently when coverage is enabled. Disable until 14173 is fixed.
+    platform_exclude: qemu_x86_coverage
     tags: kernel