test: cleanup after box-tap/trigger_yield

The test creates a space, but doesn't drop it, which leads to
box-tap/on_schema_init failure:

 | box-tap/trigger_yield.test.lua                                  [ pass ]
 | box-tap/on_schema_init.test.lua                                 [ fail ]
 | Test failed! Output from reject file box-tap/on_schema_init.reject:
 | TAP version 13
 | 1..7
 | ok - on_schema_init trigger set
 | ok - system spaces are accessible
 | ok - before_replace triggers
 | ok - on_replace triggers
 | ok - set on_replace trigger
 | ok - on_schema_init trigger works
 |
 | Last 15 lines of Tarantool Log file [Instance "app_server"][/Users/travis/build/tarantool/tarantool/test/var/002_box-tap/on_schema_init.test.lua.tarantool.log]:
 | 2019-03-06 17:00:12.057 [87410] main/102/on_schema_init.test.lua F> Space 'test' already exists

Fix this.

test/box-tap/trigger_yield.test.lua

@@ -7,8 +7,8 @@ box.cfg{
 
 fiber = require('fiber')
 
-box.schema.space.create('test', {if_not_exists = true})
-box.space.test:create_index('pk', {if_not_exists = true})
+box.schema.space.create('test')
+box.space.test:create_index('pk')
 
 box.space.test:truncate()
 
@@ -27,4 +27,5 @@ for _,f in pairs(fibers) do
     while f:status() ~= 'dead' do fiber.sleep(0.0001) end
 end
 print('done: '..box.space.test:len())
+box.space.test:drop()
 os.exit()