CI: add synchronous IO test

.gitlab-ci.yml

@@ -151,6 +151,10 @@ clang-tidy:
   variables:
     EMPER_IO_SINGLE_URING: 'true'
 
+.emper-synchronous-io:
+  variables:
+    EMPER_IO_SYNCHRONOUS: 'true'
+
 .emper-io-stealing:
   variables:
     EMPER_IO_STEALING: 'true'
@@ -412,6 +416,11 @@ test-single-uring:
     - .test
     - .emper-single-uring
 
+test-synchronous-io:
+  extends:
+    - .test
+    - .emper-synchronous-io
+
 test-pipe-sleep-strategy:
   extends:
     - .test