debian: Update the build tests

Update the build test to closer from other GNOME ones and fix some
issues/the current test failing.
- don't use a deprecated function
- handle cross build autopkgtest environment
- include a runtime test as well

Original patch by Sebastien Bacher available here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=946676
7 jobs for debian-build-test in 16 minutes and 11 seconds (queued for 6 minutes and 36 seconds)
latest
Status Job ID Name Coverage
  Build
passed #236723
librem5
build-debian-clang

00:04:04

passed #236721
librem5
build-debian-gcc

00:03:10

passed #236722
librem5
build-debian-gcc-static

00:03:08

passed #236724
librem5
build-fedora-gcc

00:03:41

 
  Docs
passed #236725
librem5
build-gtkdoc

00:02:26

 
  Test
passed #236726
librem5
unit-test

00:02:34

44.2%
 
  Package
passed #236727
librem5
package-debian

00:03:53