test.py: only access combined_tests if it exists

When the scylla source tree is only partially built,
we still may want to run the tests.

test.py builds a case cache at boot, and executes
--list-cases for that, for all built tests.

After amalgamating boost unit tests into a single
file, it started running it unconditionally, which broke
partial builds.

Hence, only use combined_tests executable if it exists.

Fixes #22038

test.py

@@ -373,7 +373,8 @@ class BoostTestSuite(UnitTestSuite):
 
     def _generate_cache(self) -> None:
         # Apply combined test only for test/boost
-        if self.name != 'boost':
+        exe_path = pathlib.Path(self.mode, "test", self.name, 'combined_tests')
+        if self.name != 'boost' or not exe_path.exists():
             return
         exe = path_to(self.mode, "test", self.name, 'combined_tests')
         res = subprocess.run(