Fix isTraceStack on newer Go versions

[sywhang]

This fixes #66 .

On newer Go versions (1.16 onwards), the top of trace stacks is no longer runtime.goparkunlock. We had a logic that relied on this being the case, which made the tests to fail with -trace flag for newer Go versions. However, to maintain backwards compatibility, we shouldn't just remove this check.

This separates out the isTraceStack into a separate file that gets conditionally compiled on different Go versions (tracestack_old.go for Go 1.15 and before and tracestack_new.go for Go 1.16 and newer) to address this issue.

Internal Ref: GO-887.

[codecov]

Codecov Report

Merging #68 (5dbd86f) into master (1e9de54) will increase coverage by 0.05%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage   93.27%   93.33%   +0.05%     
==========================================
  Files           4        6       +2     
  Lines         119      120       +1     
==========================================
+ Hits          111      112       +1     
  Misses          5        5              
  Partials        3        3              

Impacted Files	Coverage Δ
options.go	100.00% <ø> (+2.12%)	⬆️
tracestack_old.go	66.66% <66.66%> (ø)
tracestack_new.go	100.00% <100.00%> (ø)

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