Allow debug_assertions in short-benchmarks CI job (#1711)

Add debug_assertions to WASM builds in the short-benchmarks CI job.

Disallow warnings, show a full backtrace and remove the WASM output
colour to improve information from this CI step in the event of a failure.

This is motivated by a
[case](https://github.com/paritytech/polkadot-sdk/pull/1676/commits/23d64918)
where a benchmark was misconfigured in master and failing to send
notifications, but passing CI. We don't want this to fail if the problem
happens in production, but ideally we'd know from the CI if it was
misconfigured and the messages weren't getting sent, as that could
(would?) affect the weights that the benchmark generates.

Enabling debug-assertions in WASM builds for the short-benchmarks would
catch "soft" failures like this in future.