test: kernel: context: Exclude for qemu_cortex_r5 (temporary)

The `tests/kernel/context` test fails for the `qemu_cortex_r5` when the
icount emulation mode is used, because the Xilinx QEMU ignores the WFI
instruction when the icount parameter is specified.

This will be fixed in the Zephyr SDK 0.11.3 and this commit must be
reverted once the CI is updated to use this new SDK version.

For more details, see zephyrproject-rtos/sdk-ng#191.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>