executor: migrate test-infra to testify for executor/union_scan_test.go (#30525)
executor: migrate test-infra to testify for executor/union_scan_test.go (#30525)

 
c4pt0r/tidb,ngaut/tidb,kyle-wang/tidb,kyle-wang/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,c4pt0r/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,c4pt0r/tidb,pingcap/tidb,c4pt0r/tidb,pingcap/tidb,ngaut/tidb,ngaut/tidb