Mark a singleton instance as a leader manually

Otherwise 'set' and 'register' tests, which use synchronous spaces, fail
with the 'The synchronous transaction queue doesn't belong to any
instance' error.

Those tests did pass before the change [1] in tarantool, which was made
in the scope of the issue [2]. The commit land into release branches as
2.7.2-160-g7310288fa, 2.8.1-168-g806123567, 2.9.0-221-g362e9a668.

[1]: tarantool/tarantool@362e9a6
[2]: tarantool/tarantool#6034

Fixes #92

Author: Totktonada

resources/tarantool/jepsen.lua

@@ -29,6 +29,13 @@ else
 end
 
 local function bootstrap()
+    -- We don't use automatic leader election in the single instance
+    -- configuration, so we should manually mark this instance as
+    -- a leader for synchronous transactions processing.
+    if single_mode then
+        box.ctl.promote()
+    end
+
     box.schema.user.create('jepsen', {password = 'jepsen'})
     box.schema.user.grant('jepsen', 'create,read,write,execute,drop,alter,replication', 'universe')
     box.schema.user.grant('jepsen', 'read,write', 'space', '_index')