x/simulation: make signals channel buffered to avoid missing signals

[odeke-em]

Invoking:

c := make(chan os.Signal)
signal.Notify(c, signals...)

is explicitly called out as a bug in the os/signal docs
and should instead make that channel buffered lest
a signal could be lost.

Fixes #7950

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #7952 (034045d) into master (0647d89) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #7952   +/-   ##
=======================================
  Coverage   54.14%   54.14%           
=======================================
  Files         612      612           
  Lines       39029    39029           
=======================================
  Hits        21134    21134           
  Misses      15720    15720           
  Partials     2175     2175           

[alexanderbez]

utACK