tests: Widely switch to new commit/branch syntax

Lift most test cases to the new syntax, just with few exceptions:
 - tests/test_refspec/test2.yml actually stresses the mixed usage
   (but clean per repo)
 - tests/test_repo_includes references kas as external repo at a point
   where there was no new syntax yet; this needs to be migrated
   differently later on

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_layers/test.yml

@@ -1,29 +1,29 @@
 header:
-  version: 8
+  version: 14
 
 repos:
   this:
 
   kas:
     url: https://github.com/siemens/kas.git
-    refspec: master
+    branch: master
 
   kas1:
     url: https://github.com/siemens/kas.git
-    refspec: master
+    branch: master
     layers:
       meta-foo:
       meta-bar:
 
   kas2:
     url: https://github.com/siemens/kas.git
-    refspec: master
+    branch: master
     layers:
       .: excluded
 
   kas3:
     url: https://github.com/siemens/kas.git
-    refspec: master
+    branch: master
     layers:
       .:
       meta-bar: