tests: use origin node logger instead of creating new TestLogger

[JosephGoulden]

I'm using function_test_utils which are really helpful. It would be good if I could control the log level though.
I can set the log level on the nodes returned by create_chanmon_cfgs. But when calling the route_payment and other functions a new TestLogger is created. I'm wondering if it's ok to just use the logger of the origin node?

I can see that a few tests in function_tests also create new loggers for calling get_route as well. Potentially these could be modified as well, but this is not necessarily relevant to dependent projects making use of test_utils.

[codecov]

Codecov Report

Merging #1060 (02c95ee) into main (479ff90) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1060      +/-   ##
==========================================
+ Coverage   90.81%   90.83%   +0.02%     
==========================================
  Files          65       65              
  Lines       32801    32884      +83     
==========================================
+ Hits        29789    29871      +82     
- Misses       3012     3013       +1     

Impacted Files	Coverage Δ
lightning/src/ln/functional_test_utils.rs	95.11% <100.00%> (-0.02%)	⬇️
lightning-background-processor/src/lib.rs	93.99% <0.00%> (+0.05%)	⬆️
lightning/src/ln/functional_tests.rs	97.50% <0.00%> (+0.06%)	⬆️

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