tests: add integration tests for dap build

This adds integration tests for the `dap build` command to test various
behavior associated with the command. We start the build and the
integration test acts as a dap client to send requests and check that
the output is what we expect.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

dap/conn.go

@@ -6,16 +6,13 @@ import (
 	"io"
 	"sync"
 
+	"github.com/docker/buildx/dap/common"
 	"github.com/google/go-dap"
 	"github.com/pkg/errors"
 	"golang.org/x/sync/errgroup"
 )
 
-type Conn interface {
-	SendMsg(m dap.Message) error
-	RecvMsg(ctx context.Context) (dap.Message, error)
-	io.Closer
-}
+type Conn = common.Conn
 
 type conn struct {
 	recvCh <-chan dap.Message