misc/ios: deflake tests on darwin/arm{,arm64}

A retry mechanism is in place to combat the inherent flakiness of
launching iOS test binaries. Before it covered just the starting of
lldb; expand it to cover the setup steps as well. Note that the
running of the binary itself is (still) not retried, to avoid
covering over genuine bugs.

On my test device (iPhone 5S, iOS 9.3) starting lldb can take longer
than 10 seconds, so increase the timeout for that.
Furthermore, some basic steps such as setting breakpoints in lldb
can take longer than the 1 second timeout. Increase that timeout
as well, to 2 seconds.

Finally, improve the error message for when ios-deploy is not
installed.

For #14318

Change-Id: Iba41d1bd9d023575b9454cb577b08f8cae081c2a
Reviewed-on: https://go-review.googlesource.com/21072
Reviewed-by: David Crawshaw <crawshaw@golang.org>

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