Add unit tests for ds-identify, fix Ec2 bug found.

This adds several unit tests for ds-identify, and fixes a bug
in Ec2 detection that I found while writing these tests.

The method of testing is to use the ds-identify code as a shell
library. The TestDsIdentify:call basically does:

  * populate a (temp) directory with files that represent what
    ds-identify would see in /sys or other locations it reads.
  * create a file '_shwrap' that replaces the 3 programs that are executed
    in ds-identify code path. It supports setting their stdout, stderr,
    and exit code.
  * set the default policies explicitly (DI_DEFAULT_POLICY) so we can
    support testing different builtins. This is necessary because the
    Ubuntu branches patch the builtin value. If we did not explicilty set
    it, then testing there would fail.
  * execute sh to source the script and call its main.

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