SDK - Testing - Fix metadata comparison instability

[Ark-kun]

After #2044 was merged the unit tests become flaky on Python 3.5 since the serialization of the dictionaries used in the default pipeline parameter values is not deterministic.
This PR fixes the root cause of the flakiness - the tests that were comparing the serialized metadata strings instead of the structures.

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[Ark-kun]

/approve

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[Ark-kun]

/retest

[numerology]

/lgtm

[numerology]

QQ: is that possible to do something like assertDictEqual to compare the actual pipeline_spec?

[Ark-kun]

QQ: is that possible to do something like assertDictEqual to compare the actual pipeline_spec?

I did something like that first - deserialized the spec into structure so that it's compared structurally. 42492d5 Unfortunately, the problem was deeper since the default values are also serialized, so there is another layer of serialization.

One solution would be to make the default value serialization stable by sorting all keys in the deep structure, but I'm still thinking whether this is a good idea.

[numerology]

QQ: is that possible to do something like assertDictEqual to compare the actual pipeline_spec?

I did something like that first - deserialized the spec into structure so that it's compared structurally. 42492d5 Unfortunately, the problem was deeper since the default values are also serialized, so there is another layer of serialization.

One solution would be to make the default value serialization stable by sorting all keys in the deep structure, but I'm still thinking whether this is a good idea.

Got it. We definitely need to fix this first.