Add new f-string test case

Author: Adrien Chauve

test-data/unit/check-expressions.test

@@ -1167,6 +1167,7 @@ u'%s' % (u'abc',)
 f'foobar'
 f'{"foobar"}'
 f'foo{"bar"}'
+f'.{1}.'
 a: str
 a = f'foobar'
 a = f'{"foobar"}'
@@ -1178,7 +1179,6 @@ f' {1 + ""}'
 [out]
 main:2: error: Unsupported operand types for + ("int" and "str")
 main:3: error: Unsupported operand types for + ("int" and "str")
-main:3: error: Unsupported operand types for + ("str" and "int")
 
 
 -- Lambdas