Add missing UT to Static Middleware

[pafuent]

The #1718 modified how static files are handled to prevent directory traversal. On that PR, some unit test were added to echo_test.go, but those test were not implemented for the Static Middleware (static_test.go)
The idea of this issue is to implement the missing test on the Static Middleware in relation to TestEchoStatic tests.

[aldas]

@pafuent there were tests for middleware. See that patch file in comment #1718 (comment)

[pafuent]

@aldas Nice!!!
The idea of this issue was to foster contributions though the use of "good first issue" label. But if the code is already there, I guess I'll wait some more days, and then I'll make use of your patch to avoid this blind spot on our testing.

[lammel]

Covered by PR 1740. We now have tests covering static directory traversal also for the static middlerware.
Thanks @aldas