chore(e2e): check response body read error only if a body is expected

[M4tteoP]

Working on corazawaf/coraza-caddy#96 (e2e is failing I'm trying to update it with the new one), I found that Caddy properly aborts the response if at phase 3 a deny action is triggered. It means that the response body can not be read.
Caddy Log:

e2e-caddy-1  | {"level":"error","ts":1690387902.7979822,"logger":"http.handlers.waf","msg":"[client \"192.168.176.4\"] Coraza: Access denied (phase 3).  [file \"\"] [line \"10\"] [id \"103\"] [rev \"\"] [msg \"\"] [data \"\"] [severity \"emergency\"] [ver \"\"] [maturity \"0\"] [accuracy \"0\"] [hostname \"\"] [uri \"/response-headers?pass=leak\"] [unique_id \"RkQqgctUauuoQNPt\"]\n"}
e2e-caddy-1  | {"level":"error","ts":1690387902.798008,"logger":"http.handlers.reverse_proxy","msg":"aborting with incomplete response","error":"short write"}

e2e output:

e2e-tests-1  | [5/9] Running test: Denied request with a malicious response header
e2e-tests-1  | [Fail] could not read response body: unexpected EOF
e2e-tests-1  | exit status 1

This PR should fix the problem looking at the error generated by reading the response only when we actually want to check the response body.

[codecov]

Codecov Report

Patch coverage: 78.98% and project coverage change: +0.07% 🎉

Comparison is base (1b8a007) 81.51% compared to head (6839e51) 81.59%.
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #852      +/-   ##
==========================================
+ Coverage   81.51%   81.59%   +0.07%     
==========================================
  Files         158      159       +1     
  Lines        8947     9013      +66     
==========================================
+ Hits         7293     7354      +61     
- Misses       1408     1412       +4     
- Partials      246      247       +1     

Flag	Coverage Δ
default	76.64% <80.74%> (+0.47%)	⬆️
examples	25.52% <16.66%> (+<0.01%)	⬆️
ftw	46.85% <40.47%> (-0.24%)	⬇️
ftw-multiphase	48.98% <40.47%> (-0.25%)	⬇️
tinygo	74.85% <96.59%> (+2.62%)	⬆️

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Files Changed	Coverage Δ
http/e2e/cmd/httpe2e/main.go	0.00% <ø> (ø)
internal/actions/allow.go	100.00% <ø> (ø)
internal/actions/auditlog.go	72.72% <ø> (ø)
internal/actions/block.go	100.00% <ø> (ø)
internal/actions/capture.go	100.00% <ø> (ø)
internal/actions/chain.go	100.00% <ø> (ø)
internal/actions/ctl.go	92.87% <ø> (ø)
internal/actions/deny.go	84.21% <ø> (ø)
internal/actions/drop.go	84.21% <ø> (ø)
internal/actions/exec.go	70.00% <ø> (ø)
... and 32 more

... and 1 file with indirect coverage changes

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[jcchavezs]

Mentioning the example makes the case