Unit Tests Skipped
These are for the build tools stage, and should probably not be controlled by MK_TOOLCHAIN. They'd need the equivalent of MK_TOOLCHAIN_BOOTSTRAP, but probably best to just leave them on unconditionally.
gnu/usr.bin/binutils/ is only built when MK_TOOLCHAIN == yes so no need to test it here.
I'm not certain what we should be testing here and below. We want the equivalent of MK_BINUTILS_BOOTSTRAP for ELF Tool Chain. MK_BINUTILS_BOOTSTRAP isn't documented, but is enabled unless turned off by setting WITHOUT_CROSS_COMPILER or if set broken by the arch.
I might just use MK_BINUTILS_BOOTSTRAP since is effectively internal and I think expresses what we want (don't built the non-compiler part of the toolchain). lld may complicate that a bit since it's llvm based and thus will come with clang not with elftoolchain or binutils.