tests: check for negative syscalls in runBpf() and skip those tests

[mvo5]

Some syscalls are resolved to negative numbers by libseccomp. An
example is the "socket" syscall which turns into -101 on i386 and
s390x on 14.04, 17.04 and 17.10.

We cannot run the unittests against a negative number so we need to
skip the unit test on these platforms for now.

This causes a FTBFS on {zesty,artful}/{i386,s390x}
apd/blob/master/CONTRIBUTING.md)?

[zyga]

+1

[codecov-io]

Codecov Report

Merging #3850 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3850      +/-   ##
==========================================
+ Coverage   75.71%   75.71%   +<.01%     
==========================================
  Files         413      413              
  Lines       35555    35555              
==========================================
+ Hits        26921    26922       +1     
+ Misses       6730     6729       -1     
  Partials     1904     1904

Impacted Files	Coverage Δ
cmd/snap/cmd_aliases.go	95% <0%> (+1.66%)	⬆️

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[jdstrand]

-1

[jdstrand]

17.04 and 17.10 shouldn't be using socketcall on i386 and s390x. The kernel and glibc should only be using socket and failures here would indicate a regression on those releases. Three pieces are needed, the kernel, glibc and libseccomp 2.3 for this to work. On 16.04, all this was in place-- why did this regress?

References:

• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=809556 (i386 fixes)
• https://bugs.launchpad.net/ubuntu/+source/libseccomp/+bug/1554098 (s390x fixes)
• https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=x86/asm&id=9dea5dc921b5f4045a18c63eb92e84dc274d17eb (kernel commit to 4.3)
• https://bugs.launchpad.net/ubuntu/+source/libseccomp/+bug/1576066