Rollup merge of #35049 - knight42:add-test, r=alexcrichton
Rollup merge of #35049 - knight42:add-test, r=alexcrichton

Add a test for AddAssign on String

Fix #35047

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust