cmd/go: go test -cover interacts badly with cache #22652
Description
$ go clean -cache
$ go test -cover math strings
ok math 0.014s coverage: 84.2% of statements
ok strings 0.311s coverage: 98.3% of statements
$ go clean -cache
$ go test -cover math
ok math 0.014s coverage: 84.2% of statements
$ go test -cover math strings
ok math (cached) coverage: 84.2% of statements
ok strings 0.285s coverage: 98.3% of statements
$ go clean -cache
$ go test -cover strings
ok strings 0.288s coverage: 98.3% of statements
$ go test -cover math strings
# strings (testmain)
/var/folders/qk/vp4mp_hj5qlb_1_w97rgqc4r000n9d/T/go-build211189621/b047/_testmain.go:359:42: undefined: strings.GoCover_45
/var/folders/qk/vp4mp_hj5qlb_1_w97rgqc4r000n9d/T/go-build211189621/b047/_testmain.go:361:42: undefined: strings.GoCover_46
/var/folders/qk/vp4mp_hj5qlb_1_w97rgqc4r000n9d/T/go-build211189621/b047/_testmain.go:363:41: undefined: strings.GoCover_47
/var/folders/qk/vp4mp_hj5qlb_1_w97rgqc4r000n9d/T/go-build211189621/b047/_testmain.go:365:42: undefined: strings.GoCover_48
/var/folders/qk/vp4mp_hj5qlb_1_w97rgqc4r000n9d/T/go-build211189621/b047/_testmain.go:367:41: undefined: strings.GoCover_49
/var/folders/qk/vp4mp_hj5qlb_1_w97rgqc4r000n9d/T/go-build211189621/b047/_testmain.go:369:42: undefined: strings.GoCover_50
/var/folders/qk/vp4mp_hj5qlb_1_w97rgqc4r000n9d/T/go-build211189621/b047/_testmain.go:371:48: undefined: strings.GoCover_51
/var/folders/qk/vp4mp_hj5qlb_1_w97rgqc4r000n9d/T/go-build211189621/b047/_testmain.go:373:47: undefined: strings.GoCover_52
ok math 0.014s coverage: 84.2% of statements
FAIL strings [build failed]
$ ```