chore(*) isolate test suites by Wasm target

[thibaultcha]

• Remove WASI spillover from test crates compatible with the wasm32-unknown-unknown target.
• Default all tests crates to compile wasm32-wasip1 targets.
• Can set TEST_NGINX_CARGO_RUSTTARGET to override.
• Cleanup build tests framework.
• CI: add a job with wasm32-unknown-unknown target.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.56826%. Comparing base (f8d79ee) to head (5e5befa).
Report is 3 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #594         +/-   ##
===================================================
- Coverage   90.69725%   90.56826%   -0.12900%     
===================================================
  Files             51          51                 
  Lines          11072       11016         -56     
===================================================
- Hits           10042        9977         -65     
- Misses          1030        1039          +9     

Files with missing lines	Coverage Δ
src/wasm/wasi/ngx_wasi_preview1_host.c	78.08219% <ø> (ø)
src/wasm/wrt/ngx_wrt_v8.c	88.84758% <ø> (ø)
src/wasm/wrt/ngx_wrt_wasmer.c	81.86398% <ø> (+0.18459%)	⬆️
src/wasm/wrt/ngx_wrt_wasmtime.c	82.83828% <100.00000%> (+0.26946%)	⬆️

... and 31 files with indirect coverage changes

Flag	Coverage Δ
unit	90.48204% <100.00000%> (+0.06967%)	⬆️
valgrind	81.26431% <ø> (-1.08981%)	⬇️

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