topotests: skip tests when any assert fails

[rzalamena]

When an assert fails we should skip all other tests on the file. Once
a failure is detected we can't rely on the setup anymore, since most of
the tests assume the previous worked.

---

This will help tests to end earlier in case of tests failures and will only skip tests with the guard:

    if tgen.routers_have_failure():
        pytest.skip(tgen.errors) 

(this only applies to topologies using topogen)

Reported by @rwestphal

[LabN-CI]

💚 Basic BGPD CI results: SUCCESS, 0 tests failed

Results table

_	_
Result	SUCCESS git merge/5087 c8c265f
Date	10/01/2019
Start	08:30:15
Finish	08:51:49
Run-Time	21:34
Total	1815
Pass	1815
Fail	0
Valgrind-Errors	0
Valgrind-Loss	0
Details	vncregress-2019-10-01-08:30:15.txt
Log	autoscript-2019-10-01-08:31:09.log.bz2
Memory	420 426 360

For details, please contact louberger

[rwestphal]

LGTM, thanks Rafael!

[NetDEF-CI]

Continuous Integration Result: SUCCESSFUL

Congratulations, this patch passed basic tests

Tested-by: NetDEF / OpenSourceRouting.org CI System

CI System Testrun URL: https://ci1.netdef.org/browse/FRR-FRRPULLREQ-9100/

This is a comment from an automated CI system.
For questions and feedback in regards to this CI system, please feel free to email
Martin Winter - mwinter (at) opensourcerouting.org.

---

CLANG Static Analyzer Summary

• Github Pull Request 5087, comparing to Git base SHA 698603c

No Changes in Static Analysis warnings compared to base

1 Static Analyzer issues remaining.

See details at
https://ci1.netdef.org/browse/FRR-FRRPULLREQ-9100/artifact/shared/static_analysis/index.html