test: Use linux-aio backend again on seastar-based tests

Since mid December, tests started failing with ENOMEM while
submitting I/O requests.

Logs of failed tests show IO uring was used as backend, but we
never deliberately switched to IO uring. Investigation pointed
to it happening accidentaly in commit 1bac6b75dc,
which turned on IO uring for allowing native tool in production,
and picked linux-aio backend explicitly when initializing Scylla.
But it missed that seastar-based tests would pick the default
backend, which is io_uring once enabled.

There's a reason we never made io_uring the default, which is
that it's not stable enough, and turns out we made the right
choice back then and it apparently continue to be unstable
causing flakiness in the tests.

Let's undo that accidental change in tests by explicitly
picking the linux-aio backend for seastar-based tests.
This should hopefully bring back stability.

Refs #21968.

Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>

Closes scylladb/scylladb#22695

test.py

@@ -390,6 +390,7 @@ class UnitTestSuite(TestSuite):
         super().__init__(path, cfg, options, mode)
         # Map of custom test command line arguments, if configured
         self.custom_args = cfg.get("custom_args", {})
+        self.extra_cmdline_options = cfg.get("extra_scylla_cmdline_options", [])
         # Map of tests that cannot run with compaction groups
         self.all_can_run_compaction_groups_except = cfg.get("all_can_run_compaction_groups_except")
 
@@ -401,6 +402,10 @@ class UnitTestSuite(TestSuite):
         test = UnitTest(self.next_id((shortname, self.suite_key)), shortname, suite, args)
         self.tests.append(test)
 
+    def prepare_arg(self, arg):
+        extra_cmdline_options = ' '.join(self.extra_cmdline_options)
+        return f'{arg} {extra_cmdline_options}'
+
     async def add_test(self, shortname, casename) -> None:
         """Create a UnitTest class with possibly custom command line
         arguments and add it to the list of tests"""
@@ -413,7 +418,7 @@ class UnitTestSuite(TestSuite):
         args = self.custom_args.get(shortname, ["-c2 -m2G"])
         args = merge_cmdline_options(args, self.options.extra_scylla_cmdline_options)
         for a in args:
-            await self.create_test(shortname, casename, self, a)
+            await self.create_test(shortname, casename, self, self.prepare_arg(a))
 
     @property
     def pattern(self) -> str:
@@ -531,13 +536,12 @@ class BoostTestSuite(UnitTestSuite):
         # Skip tests which are not configured, and hence are not built
         if os.path.join("test", self.name, execname if combined_test else shortname) not in self.options.tests:
                 return
-
         # Default seastar arguments, if not provided in custom test options,
         # are two cores and 2G of RAM
         args = self.custom_args.get(shortname, ["-c2 -m2G"])
         args = merge_cmdline_options(args, self.options.extra_scylla_cmdline_options)
         for a in args:
-            await self.create_test(shortname, casename, self, a)
+            await self.create_test(shortname, casename, self, self.prepare_arg(a))
 
     def junit_tests(self) -> Iterable['Test']:
         """Boost tests produce an own XML output, so are not included in a junit report"""

test/boost/suite.yaml

@@ -1,4 +1,6 @@
 type: boost
+extra_scylla_cmdline_options:
+    - '--reactor-backend linux-aio'
 # A list of long tests, which should be started early
 run_first:
     - index_with_paging_test

test/raft/suite.yaml

@@ -1 +1,3 @@
 type: boost
+extra_scylla_cmdline_options:
+  - '--reactor-backend linux-aio'