Update to gotest.tools/v3

[mem]

Signed-off-by: Marcelo E. Magallon marcelo@sylabs.io

Description of the Pull Request (PR):

Write your description of the PR here. Be sure to include as much background,
and details necessary for the reviewers to understand exactly what this is
fixing or enhancing.

This fixes or addresses the following GitHub issues:

• Fixes Issue Importing from Projects Using Go 1.13 #4381

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• Read the Guidelines for Contributing, and this PR conforms to the stated requirements.
• Added changes to the CHANGELOG if necessary according to the Contribution Guidelines
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Attn: @singularity-maintainers

[tri-adam]

LGTM. Tested with:

+replace github.com/sylabs/singularity => github.com/mem/singularity v0.0.0-20190904202734-c11c37dab640

and can confirm I'm no longer seeing #4381. Thank you!