selftests/arm64: Print missing MTE TAP headers

selftests/arm64: Print missing MTE TAP headers

Most MTE tests set a TAP plan and emit results without first printing
the TAP version header. Direct execution therefore starts with a plan
such as "1..20" instead of "TAP version 13".

The problem is particularly visible in the GCR_EL1 context-switch test.
It prints its plan before forking 1,024 child processes. When stdout is
fully buffered, the plan remains in the stdio buffer. Each child inherits
the pending "1..1" line and flushes its copy from exit(), producing
repeated plan lines.

ksft_print_header() prints the TAP header and enables line buffering.
Call it in every MTE test that is missing it. In the GCR_EL1 test, call
it before the plan so the plan is flushed before the children are
forked. In the remaining tests, call it before setup and prerequisite
checks so early failures and whole-test skips also retain the header.

Fixes: 29f080881601 ("kselftest/arm64: check GCR_EL1 after context switch")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>