[doc] contributing-testing.md should be updated because of "cargo test --all"

[misapprehand]

cargo test --all is broken by

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> crates/lightbeam/src/lib.rs:1:19
  |
1 | #![cfg_attr(test, feature(test))]
  |                   ^^^^^^^^^^^^^

error: aborting due to previous error

it should be updated as described in scrips/test-all.sh

# TODO: lightbeam currently requires rust nightly, so don't try to run the
# tests here. Name all the other packages, rather than using --all. We'll
# run the lightbeam tests below if nightly is available.
#RUST_BACKTRACE=1 cargo test --all
RUST_BACKTRACE=1 cargo test \
  --package wasmtime-cli \
  --package wasmtime \
  --package wasmtime-wasi \
  --package wasmtime-wast \
  --package wasmtime-debug \
  --package wasmtime-environ \
  --package wasmtime-runtime \
  --package wasmtime-jit \
  --package wasmtime-obj \
  --package wiggle \
  --package wiggle-generate \
  --package wiggle-test \
  --package wiggle-macro \
  --package wasi-common

[alexcrichton]

Lightbeam was removed in #3390 as explained in RFC 14, so I'm going to close this.