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_commands/test-invalid.yml

@@ -1,7 +1,7 @@
 header:
-  version: 8
+  version: 14
 
 repos:
   kas_invalid:
     url: https://example.com/kas.git
-    refspec: this-ref-is-invalid
+    branch: this-ref-is-invalid

tests/test_commands/test.yml

@@ -1,13 +1,13 @@
 header:
-  version: 8
+  version: 14
 
 repos:
   this:
 
   kas_1.0:
     url: https://github.com/siemens/kas.git
-    refspec: 907816a5c4094b59a36aec12226e71c461c05b77
+    commit: 907816a5c4094b59a36aec12226e71c461c05b77
 
   kas_1.1:
     url: https://github.com/siemens/kas.git
-    refspec: e9ca55a239caa1a2098e1d48773a29ea53c6cab2
+    commit: e9ca55a239caa1a2098e1d48773a29ea53c6cab2