Simplify code that executes scamper

[SaiedKazemi]

This commit uses the os/exec package to execute an instance of the scamper command. Previously, the m-lab/go/shx package was used because executing the scamper command was more elaborate and required a 3-stage pipeline.

Changes were tested locally with "go test" and also with "docker-compose"

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 440

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 96.86%

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Totals
Change from base Build 438:	0.01%
Covered Lines:	617
Relevant Lines:	637

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💛 - Coveralls

[SaiedKazemi]

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tracer/scamper.go, line 178 at r1 (raw file):

Previously, cristinaleonr (Cristina Leon) wrote…

Do we want to add some context to this line we're printing out (like above)?

I don't think so because this is the standard error of the command (as opposed to a function's error message) which doesn't require additional context and is printed verbatim. The lines above it already provide context and details.

[SaiedKazemi]

Thanks for your review. Please see my explanation.

[SaiedKazemi]

Thanks.