checkstyle: batch mdformat into single invocation

test_markdown_style was spawning a bash+diff+mdformat subprocess for
each of the 66 markdown files individually (23s). Pass all files to
mdformat --check in one call (~4s).

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

tests/integration_tests/style/test_markdown.py

@@ -11,26 +11,15 @@ def test_markdown_style():
     """
     Test that markdown files adhere to the style rules.
     """
-    # Get all *.md files from the project
     md_files = list(utils_repo.git_repo_files(root="..", glob="*.md"))
-
-    # Assert if somehow no markdown files were found.
     assert len(md_files) != 0
 
-    needs_format = False
-
-    # Run commands
-    for md_file in md_files:
-        rc, output, _ = utils.run_cmd(
-            f"bash -c 'diff -u --color {md_file} <(mdformat - < {md_file})'",
-        )
-        if rc != 0:
-            print(output)
-            needs_format = True
+    file_args = " ".join(str(f) for f in md_files)
+    rc, output, _ = utils.run_cmd(f"mdformat --check {file_args}")
 
     assert (
-        not needs_format
-    ), "Some markdown files need formatting. Either run `./tools/devtool sh mdformat .` in the repository root, or apply the above diffs manually."
+        rc == 0
+    ), f"Some markdown files need formatting. Either run `./tools/devtool sh mdformat .` in the repository root, or apply the diffs manually.\n{output}"
 
 
 def test_markdown_internal_links():