test: py: Use run_ut() helper for vbe_test_fixup

Split the base_script to separate the ut command from the U-Boot command
sequence. This allows using run_ut() for the vbe_test_fixup test,
reducing duplication and improving consistency.

Cover-letter:
test: py: Convert more tests to use run_ut() helper
This series converts additional Python tests to use the run_ut() helper.
This reduces duplication and makes the tests more concise and consistent.

The ext4l tests are straightforward conversions, while test_vbe.py
requires splitting the U-Boot command script to separate the ut command.
END

Series-to: concept
Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

test/py/tests/test_vbe.py

@@ -87,7 +87,6 @@ bootm loados
 bootm prep
 fdt addr
 fdt print
-ut -f bootstd vbe_test_fixup_norun
 '''
 
 @pytest.mark.boardspec('sandbox')
@@ -117,9 +116,9 @@ def test_vbe_os_request(ubman):
     cmd = base_script % params
 
     with ubman.log.section('Kernel load'):
-        output = ubman.run_command_list(cmd.splitlines())
+        ubman.run_command_list(cmd.splitlines())
 
-    assert 'failures: 0' in output[-1]
+    ubman.run_ut('bootstd', 'vbe_test_fixup')
 
 @pytest.mark.boardspec('sandbox')
 def test_vbe_extlinux_fit_no_oem(ubman):