elf/tests: remove unnecessary qualification

[00xc]

Fix the following clippy warning on the nightly toolchain:

error: unnecessary qualification
    --> kernel/src/elf/mod.rs:2539:29
     |
2539 |         assert_eq!(res, Err(crate::elf::ElfError::InvalidPhdrSize));
     |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: requested on the command line with `-D unused-qualifications`
help: remove the unnecessary path segments
     |
2539 -         assert_eq!(res, Err(crate::elf::ElfError::InvalidPhdrSize));
2539 +         assert_eq!(res, Err(ElfError::InvalidPhdrSize));
     |

[00xc]

For context these need to be fixed for Miri to properly run, as it uses the nightly toolchain and we cause a build error on some warnings like this one.