fix(run): let parameters expand in the command shell, not the ZUnit shell

[ss-o]

Adds $ to the requoting escape set in run. Two characters of source, plus two regression tests.

The defect

run builds its command as text and evals it, requoting every word that is not a shell operator:

${MATCH//(#b)([\"\`\\])/\\${match[1]}}

", backtick and backslash are escaped. $ is not, so a $ inside a quoted word is expanded by the eval in the ZUnit shell and never reaches the command:

run zsh -f -c 'typeset inner=INNER_VALUE; print -r -- "got=[${inner}]"'

ZUnit	Result
zdharma/zunit v0.8.2	got=[INNER_VALUE]
this fork, before	got=[]
this fork, after	got=[INNER_VALUE]

Upstream is unaffected because it never evals: output=$("${cmd[@]}" 2>&1).

Why not just adopt upstream's approach

The eval is deliberate and earns its place. dont_quote is what lets run accept redirections, ;, |, && and var=... assignments, which upstream cannot handle, and the leading newline is what puts a line number into run:1: command not found: .... Both behaviours have tests here. The narrow fix keeps them.

Why it mattered

The failure is silent. The parameter empties rather than erroring, so assertions fail for reasons that look unrelated to quoting, or pass vacuously.

Found via z-shell/zd: tests/snippets.zunit test 4 fails under this fork with no such file or directory: "/._zi/mirror-backend" because ${object_path} empties, while the same test passes in that repo's CI, which installs upstream v0.8.2. With this change the suite goes from 3/4 to 4/4 locally.

Verification

Both new tests in tests/run.zunit were run against the unmodified source and observed failing, then passing with the change:

against unmodified source:
  not ok 4 - Command parameters expand in the command shell, not the ZUnit shell
  not ok 5 - A var= assignment reaches the command shell intact

with the change:
  1..125   (no failures)

Test non-existent command still asserts run:[0-9]+: command not found:, so the line-number behaviour the newline exists for is unchanged. Suite goes from 123 to 125 passing.

Closes #24