kselftest/arm64: Fix check_buffer_fill test

kselftest/arm64: Fix check_buffer_fill test

The check_buffer_fill test reports the error below because the test
plan is not declared correctly:

  # Planned tests != run tests (0 != 20)

Fix the test adding the correct test plan declaration.

Fixes: e9b60476bea0 ("kselftest/arm64: Add utilities and a test to validate mte memory")
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Gabor Kertesz <gabor.kertesz@arm.com>
Cc: Amit Daniel Kachhap <amit.kachhap@arm.com>
Link: https://lore.kernel.org/r/20201026121248.2340-2-vincenzo.frascino@arm.com
Signed-off-by: Will Deacon <will@kernel.org>