fix: isolate integration testserver into a nested module (break celeris↔loadgen dep cycle)

[FumingPower3925]

Problem

Perpetual dependabot churn from a celeris ↔ loadgen dependency cycle:

• loadgen's main module required github.com/goceleris/celeris — solely because of internal/integrationtest/testserver/main.go (a package main test target spinning up a real celeris server for the h2c integration matrix). go mod why celeris confirmed that single path.
• Because the require sat in the released module, every celeris release forced a dependabot bump + a loadgen re-release. And the celeris repo's test/perfmatrix module depends on loadgen, so that re-release bounced back as a dependabot PR inside the celeris repo → self-perpetuating loop.

Fix

Carve the testserver into its own nested module so loadgen's released module no longer requires celeris:

• go mod why celeris → "main module does not need package celeris".
• The nested module pins celeris independently and can be bumped freely — bumping it never re-releases loadgen, so the cycle is broken at its only closing edge.
• (celeris was already invisible to loadgen's consumers — it lived under internal/, pruned by Go 1.17+ module-graph pruning. This stops loadgen re-releasing on every celeris release.)

Changes

• New internal/integrationtest/testserver/go.mod (+go.sum) requiring celeris.
• Removed celeris (+ orphaned indirects) from the root go.mod via go mod tidy — root now requires only hdrhistogram-go + x/net.
• integration_h2c_test.go builds the testserver from inside its module dir (cmd.Dir) instead of by path — no go.work, so release go build ./... stays clean (nested module excluded from ./...).
• dependabot: watch the nested module (keeps its celeris pin fresh; harmless — no longer triggers a loadgen release).

Verified

go build/vet/test ./... (root, celeris-free), testserver builds in its module, integration test compiles (-tags integration), gofmt clean.

Paired with a celeris-repo PR that deletes the redundant test/perfmatrix bench harness (the forward edge). Either cut breaks the cycle; both = fully clean.