rename: Modify existing test to show potential bug.

Error message seen from test:

$ go test
Renamed 2 occurrences in 1 file in 1 package.
Renamed 2 occurrences in 1 file in 1 package.
Renamed 2 occurrences in 2 files in 2 packages.
Renamed 2 occurrences in 1 file in 1 package.
Renamed 2 occurrences in 1 file in 1 package.
Renamed 4 occurrences in 1 file in 1 package.
Renamed 4 occurrences in 1 file in 1 package.
Renamed 4 occurrences in 1 file in 1 package.
/go/src/main/0.go:9:6: z declared but not used
/go/src/main/0.go:9:6: z declared but not used
Renamed 2 occurrences in 1 file in 1 package.
Renamed 2 occurrences in 1 file in 1 package.
Renamed 2 occurrences in 1 file in 1 package.
Renamed 3 occurrences in 2 files in 2 packages.
Renamed 5 occurrences in 1 file in 1 package.
Renamed 5 occurrences in 1 file in 1 package.
Renamed 3 occurrences in 1 file in 1 package.
Renamed 2 occurrences in 1 file in 1 package.
Renamed 1 occurrence in 1 file in 1 package.
Renamed 2 occurrences in 1 file in 1 package.
Renamed 1 occurrence in 1 file in 1 package.
Renamed 2 occurrences in 1 file in 1 package.
--- FAIL: TestRewrites (0.02s)
        rename_test.go:1009: -from "main.y" -to "x": unexpected error: couldn't load packages due to errors: main
        rename_test.go:1009: -from "main.f::y" -to "x": unexpected error: couldn't load packages due to errors: main
FAIL
exit status 1
FAIL    golang.org/x/tools/refactor/rename      0.056s