expr depends on oniguruma, a C library, so cross-compiling it needs a
WASI sysroot: install wasi-sdk in the jobs that build wasm targets and
point CC/AR at it.
With that in place, neither expr nor test has to be excluded from the
wasmtime unit test run, and test_expr::/test_test:: can join the
integration tests run against the WASI binary.
Sylvestre Ledru
9340caf1a6