Autoharness include/exclude pattern does not support whitespace

[tautschnig]

See https://github.com/model-checking/verify-rust-std/actions/runs/14673784039/job/41186052310?pr=334: when trying to use something like --include-pattern "num::<impl i8>::wrapping_sh"
our splitting of RUSTFLAGS on whitespace in base_rustc_flags falls over.

[tautschnig]

One way to work around the RUSTFLAGS parsing problem is to add regular expression support when handling --include-pattern (and --exclude-pattern) for we could then just use . or [[:space:]] instead of the literal whitespace.