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.

.gitignore

@@ -37,6 +37,9 @@ man/aptly.1.ronn
 system/env/
 
 # created by make build for release artifacts
+VERSION
+aptly.test
+
 build/
 
 pgp/keyrings/aptly2*.gpg