Capture coverage of integration tests

To capture the coverage also for the integration tests,
a test only executing the cmd.Run function is used.

The test always exits with code 0 and prints the
real exit code to stdout. Otherwise no coverage
report is generated.

Those changes enable a more accurate coverage report
for future contributions.

api/api_test.go

@@ -2,8 +2,6 @@ package api
 
 import (
 	"encoding/json"
-	ctx "github.com/aptly-dev/aptly/context"
-	"github.com/gin-gonic/gin"
 	"io"
 	"io/ioutil"
 	"net/http"
@@ -11,6 +9,9 @@ import (
 	"os"
 	"testing"
 
+	ctx "github.com/aptly-dev/aptly/context"
+	"github.com/gin-gonic/gin"
+
 	"github.com/smira/flag"
 
 	. "gopkg.in/check.v1"