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.

go.mod

@@ -1,6 +1,6 @@
 module github.com/aptly-dev/aptly
 
-go 1.15
+go 1.16
 
 require (
 	github.com/AlekSi/pointer v1.0.0