amdgpu: Provide more specific error message if non-privileged user runs amdgpu_test

Before this change, the error message is:
"WARNING - Suite initialization failed..."
People might think this is a driver problem.

Tested with non-privileged user. Now the error message is like:
...
Error:Permission denied. Hint:Try to run this test program as root.
WARNING - Suite initialization failed for 'Basic Tests'.
...

Tested as root with no regression.

amdgpu_test uses CUnit. CUnit outputs warning message to stdout.
To be consistent, this commit outputs error message to stdout.

v2: Use strerror instead of %m. %m is a GNU C Library extension.
v3: Limit code and commit message within 80 characters per line.
    Update commit message.
    Remove a space before starting parenthesis in function call.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>