tests: fix shellcheck issues in `ech_tests.sh`

Add double-quotes where missing.

Follow-up to a362962b7289ec02b412890c9515657cf0ed50ac #11922
Closes #13382

tidy-up: whitespace [ci skip]

TLS: add support for ECH (Encrypted Client Hello)

An EXPERIMENTAL feature used with CURLOPT_ECH and --ech.

Closes #11922