[NFCI][mlir][Tests] Rename identifiers minor/major to avoid clashes with system headers

Identifiers major and minor are often already taken in POSIX systems due
to their presence in <sys/types.h> as part of the makedev library
function.

This causes compilation failures on FreeBSD and Linux systems with glibc
<2.28.

This change renames the identifiers to major_/minor_.

Differential Revision: https://reviews.llvm.org/D156683