Fix validation for trees within build_configs

Commit c00b656 ("(tests/validator) Improve build_configs verification")
added validation of the "tree" and "branch" attributes to the build_configs
property.

However, if a referenced tree is missing, the validation code will
terminate with an error as the variable used is not a dictionary but
a string type, and as such, the .get() method cannot be called.

Thus, fix which variables are referenced and the order of validation for
the "tree" and "branch" attributes to validate their presence within
a given tree object first.

While at it, update the accompanying code comment.

Signed-off-by: Krzysztof Wilczyński <[email protected]>

Author: kwilczynski

Diff for: tests/validate_yaml.py

@@ -100,25 +100,25 @@ def validate_unused_jobs(data):
 
 def validate_build_configs(data):
     '''
-    Each entry in build_configs have a tree parameter
-    This tree should exist in the trees: section
+    Each entry in build_configs must have a tree and branch attribute
+    When referenced, a given tree must exist in the trees: section
     '''
     build_configs = data.get('build_configs')
     trees = data.get('trees')
     for entry in build_configs:
-        if build_configs[entry].get('tree') not in trees.keys():
-            raise yaml.YAMLError(
-                f"Tree {entry.get('tree')} not found in trees"
-            )
-        # each build config must have fields tree and branch
-        if not build_configs[entry].get('tree'):
+        tree = build_configs[entry].get('tree')
+        if not tree:
             raise yaml.YAMLError(
                 f"Tree not found for build config: {entry}'"
             )
         if not build_configs[entry].get('branch'):
             raise yaml.YAMLError(
                 f"Branch not found for build config: {entry}'"
             )
+        if not tree in trees.keys():
+            raise yaml.YAMLError(
+                f"Tree {tree} not found in trees"
+            )
 
 def validate_unused_trees(data):
     '''