selftests: clarify common error when running gup_test

The gup_test binary will fail showing only the output of perror("open") in
the case that /sys/kernel/debug/gup_test is not found. This will almost
always be due to CONFIG_GUP_TEST not being set, which enables
compilation of a kernel that provides this file.

Add a short error message to clarify this failure and point the user to
the solution.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Joel Savitz <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Nico Pache <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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