[Tracing] Refactor DD_TAGS test cases for clarity

[zacharycmontoya]

Motivation

The tracing libraries are expected to parse DD_TAGS and assign them as span tags. Since this is both an agent setting and a tracer setting, the parsing logic is not trivial.

Changes

This PR refactors the test cases to indicate what the single DD_TAGS environment variable is and the resulting pairs of tags set on the spans.

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[bm1549]

Should we also add some test scenarios for keys/values as numbers, booleans, special characters, and other edge cases?

Suggested change

	"env:test,aKey:aVal bKey:bVal cKey:": [("env", "test"), ("aKey", "aVal bKey:bVal cKey:")],
	"env:test,aKey:aVal bKey:bVal cKey:": [("env", "test"), ("aKey", "aVal bKey:bVal cKey:")],
	"numKey:1,1:numVal,boolKey:true,true:boolVal": [("numKey", "1"), ("1", "numVal"), ("boolKey", "true"), ("true", "boolVal") ],
	"": [],
	"sp%key:value,key:sp#value": [], # I'm not sure what belongs here

[zacharycmontoya]

Yes, but I would prefer to do this in a separate PR. Some of the existing test cases are confusing to me, so I want to revisit the existing test cases and add to them as we build consensus

[bm1549]

SGTM