kselftest/arm64: Set test names prior to starting children

Since we now flush output immediately on starting children we should ensure
that the child name is set beforehand so that any output that does get
flushed from the newly created child has the name of the child attached.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221124120722.150988-1-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>