Print StringToInt and StringToInt64 defaults in sorted order

[SulimanAbdulrazzaq]

#365 made stringToStringValue.String() sort its keys, so a StringToString flag's (default [...]) text in FlagUsages() is stable between runs (#362). StringToInt and StringToInt64 never got the same change. Their String() methods still range over the map directly, so a flag with more than one default key prints them in Go's randomized map order:

f := pflag.NewFlagSet("test", pflag.ContinueOnError)
f.StringToInt("s2i", map[string]int{"c": 3, "a": 1, "d": 4, "b": 2}, "usage")
f.Lookup("s2i").Value.String() // e.g. "[a=1,d=4,b=2,c=3]", different from run to run

This is the problem #362 described: --help output, and any usage text or docs generated from it, changes on every run.

This PR sorts the keys in both String() methods, the same way string_to_string.go does. sort.Strings keeps it compatible with the Go 1.12 job in CI.

Tests

TestS2IUsageSorted and TestS2I64UsageSorted set up a four-key default and check, 100 times, that String() returns [a=1,b=2,c=3,d=4] and that FlagUsages() contains (default [a=1,b=2,c=3,d=4]).

• Without the change, both fail on the first iteration, for example expected String() to be "[a=1,b=2,c=3,d=4]" but got: "[c=3,a=1,d=4,b=2]".
• With it, they pass, and so does go test -race -v ./....
• go vet ./... is clean and gofmt reports no changes.

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[SulimanAbdulrazzaq]

@tomasaschan when you have a moment, could one of you approve the workflow runs and take a look at this one? No checks have run on the current head yet.

[tomasaschan]

The influx of PRs, especially automated ones, to this repo has been big enough that I have stopped interacting with any PRs that don't show the CLA is signed. Do that, and it gets on the review queue.