scripts: west: Fix sysbuild overwrite by test item parameter

When sysbuild is enabled globally and running west build with a
selected test scenario, the sysbuild setting was being overwritten
with False when sysbuild was not explicitly set in the test YAML file.

Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>

scripts/west_commands/build.py

@@ -354,7 +354,7 @@ class Build(Forceable):
                 self.die(f"Test item {test_item} not found in {yf}")
             item = tests.get(test_item)
 
-            sysbuild = False
+            sysbuild = None
             extra_dtc_overlay_files = []
             extra_overlay_confs = []
             extra_conf_files = []
@@ -417,7 +417,8 @@ class Build(Forceable):
 
                     self.args.cmake_opts.extend(args)
 
-            self.args.sysbuild = sysbuild
+            if sysbuild is not None:
+                self.args.sysbuild = sysbuild
 
         if found_test_metadata:
             args = []