test/security: Fix test corner case

[chu11]

Check for EACCES depends on system setup and errno may be something
else such as EROFS or EPERM. Genericize test to simply check for
failure.

[codecov-io]

Codecov Report

Merging #1029 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1029      +/-   ##
==========================================
- Coverage   77.91%   77.87%   -0.04%     
==========================================
  Files         150      150              
  Lines       25494    25494              
==========================================
- Hits        19863    19853      -10     
- Misses       5631     5641      +10

Impacted Files	Coverage Δ
src/common/libflux/info.c	76.11% <0%> (-2.99%)	⬇️
src/common/libflux/request.c	89.04% <0%> (-1.37%)	⬇️
src/common/libflux/rpc.c	91.17% <0%> (-1.11%)	⬇️
src/broker/module.c	82.15% <0%> (-0.29%)	⬇️
src/common/libflux/dispatch.c	83.15% <0%> (-0.28%)	⬇️
src/common/libflux/handle.c	85.52% <0%> (-0.26%)	⬇️
src/common/libflux/message.c	80.77% <0%> (-0.13%)	⬇️

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[coveralls]

Changes Unknown when pulling 7d891ae on chu11:issue1028 into ** on flux-framework:master**.

[garlick]

Thanks @chu11, looks good.