Linux command output display request as a part of goss command test result.

[shan100github]

Describe the feature:
Refer: #557
Currently, file comparison/validation can be performed through goss command. Failure details are not displayed during command execution.

Sample execution of goss command to diff /tmp/file1 /tmp/file2

command:
  diff:
    exit-status: 0
    exec: "diff /tmp/file1 /tmp/file2"

during the success scenarios it's fine, But the output displayed during failure scenarios is not providing details about failure.

on failure following output is displayed by goss.

$>goss v -f tap
1..1
not ok 1 - Command: diff: exit-status: doesn't match, expect: [0] found: [1]

Solution suggestion
During linux command diff /tmp/file1 /tmp/file2execution in terminal displays following content. If we can display following content as goss test result it will be easier for enduser to know failures

$>diff /tmp/file1 /tmp/file2
2c2
< am in file1
---
> am in file2

Describe alternatives considered

No alternatives considered currently, using goss command to perform file diff.

[petemounce]

Maybe use a package to generate diffs; https://godoc.org/github.com/pmezard/go-difflib/difflib I think. Windows doesn't natively provide that tool :(

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[aelsabbahy]

consolidating with #483