[PR Validation] Let markers checker match marker from whole text inst…

…ead of single line (#16571)

In PR #16069, the PR validation pipeline failed on error: Please add mark pytest.mark.topology in script bgp/test_ipv6_bgp_scale.py.

After debugging, I found that the pipeline match the pattern per line, so it will not match the marker in format:

pytestmark = [
    pytest.mark.topology(
        't0-isolated-d2u254s1', 't0-isolated-d2u254s2', 't0-isolated-d2u510',
        't1-isolated-d254u2s1', 't1-isolated-d254u2s2', 't1-isolated-d510u2'
    )
]
However, If I write all mark in same line, it will exceed the maximum line length, so I raise this PR to let mark checker match pattern from whole script instead of lines.

What is the motivation for this PR?
In summary section.

How did you do it?
let mark checker match pattern from whole script instead of lines.

How did you verify/test it?
trigger pipeline by this PR.
run script locally.
Any platform specific information?
NA

.azure-pipelines/markers_check/markers_check.py

@@ -35,12 +35,10 @@ def collect_scripts_without_topology_markers():
         script_name = s[len(location) + 1:]
         try:
             with open(s, 'r') as script:
-                for line in script:
-                    # Get topology type of script from marker `pytest.mark.topology`
-                    match = pattern.search(line)
-                    if match:
-                        has_markers = True
-                        break
+                match = pattern.search(script.read())
+                if match:
+                    has_markers = True
+                    break
 
             if not has_markers:
                 scripts_without_marker.append(script_name)