Addressed a data race condition in the server tests

[kpumuk]

Addresses a data race in the server tests.

To reproduce:

go test ./... -race

Output before the change:

==================
WARNING: DATA RACE
Read at 0x00c000536398 by goroutine 589:
  github.com/basecamp/kamal-proxy/internal/server.TestTarget_DrainRequestsThatCompleteWithinTimeout.func1()
      /Users/dmytro/work/github/kamal-proxy/internal/server/target_test.go:246 +0x40
  net/http.HandlerFunc.ServeHTTP()
      /Users/dmytro/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:2220 +0x48
  net/http.serverHandler.ServeHTTP()
      /Users/dmytro/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:3210 +0x2a8
  net/http.(*conn).serve()
      /Users/dmytro/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:2092 +0xe58
  net/http.(*Server).Serve.gowrap3()
      /Users/dmytro/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:3360 +0x4c

Previous write at 0x00c000536398 by goroutine 590:
  github.com/basecamp/kamal-proxy/internal/server.TestTarget_DrainRequestsThatCompleteWithinTimeout.func1()
      /Users/dmytro/work/github/kamal-proxy/internal/server/target_test.go:246 +0x50
  net/http.HandlerFunc.ServeHTTP()
      /Users/dmytro/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:2220 +0x48
  net/http.serverHandler.ServeHTTP()
      /Users/dmytro/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:3210 +0x2a8
  net/http.(*conn).serve()
      /Users/dmytro/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:2092 +0xe58
  net/http.(*Server).Serve.gowrap3()
      /Users/dmytro/go/pkg/mod/golang.org/[email protected]/src/net/http/server.go:3360 +0x4c

[kevinmcconnell]

Thanks for the fix, @kpumuk!