hwmonio: Add injection point to test internal behavior

With the complexities of the various behaviors within the read() method
of HwmonIO, introduce an injection point for testing. There is a
defau

hwmonio: Add injection point to test internal behavior

With the complexities of the various behaviors within the read() method
of HwmonIO, introduce an injection point for testing. There is a
default available, and therefore this is a surgical change that only
impacts future tests.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I1ead56c7fe1a2f87ebf316488e68f435a41c9d19

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