Test seL4-independent runtime crates outside of seL4

[nspin]

Either using the std and test, or, for lower-level crates such as sel4-panicking, using a new low-level test harness that calls into Linux only for printing and aborting.

• Use target.<triple>.runner in ./cargo/config.toml for cross-platform testing.
• For non-baremetal targets (where baremetal means *-sel4-*.json), use *-musl* triples to avoid having to deal with dynamic linking in QEMU emulation.
• Add scripts that run all of these tests with all relevant configurations outside of Nix.