Fix ansible-test coverage stub memory usage. (#53652)

Split coverage stub files into multiple parts based on the number
of lines in files included in each stub. This will keep memory usage
down to avoid hanging on Shippable.

test/runner/lib/cover.py

@@ -55,7 +55,28 @@ def command_coverage_combine(args):
         sources = []
 
     if args.stub:
-        groups['=stub'] = dict((source, set()) for source in sources)
+        stub_group = []
+        stub_groups = [stub_group]
+        stub_line_limit = 500000
+        stub_line_count = 0
+
+        for source in sources:
+            with open(source, 'r') as source_fd:
+                source_line_count = len(source_fd.read().splitlines())
+
+            stub_group.append(source)
+            stub_line_count += source_line_count
+
+            if stub_line_count > stub_line_limit:
+                stub_line_count = 0
+                stub_group = []
+                stub_groups.append(stub_group)
+
+        for stub_index, stub_group in enumerate(stub_groups):
+            if not stub_group:
+                continue
+
+            groups['=stub-%02d' % (stub_index + 1)] = dict((source, set()) for source in stub_group)
 
     for coverage_file in coverage_files:
         counter += 1