Expose test_process_background_events

[devrandom]

Needed for functional tests outside the crate, now that timer_tick_occured is no longer a reasonable approximation (results in spurious channel update messages).

[codecov]

Codecov Report

Merging #938 (29bad58) into main (77bdc32) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #938   +/-   ##
=======================================
  Coverage   90.56%   90.57%           
=======================================
  Files          60       60           
  Lines       30698    30698           
=======================================
+ Hits        27803    27804    +1     
+ Misses       2895     2894    -1     

Impacted Files	Coverage Δ
lightning/src/ln/channelmanager.rs	83.15% <100.00%> (ø)
lightning/src/ln/functional_tests.rs	97.18% <0.00%> (+0.01%)	⬆️

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