Merge #18305: test: Explain why test logging should be used

ffff9dc test: Explain why test logging should be used (MarcoFalke)

Pull request description:

  Background is that some tests don't have any `self.log` call at all. Thus there are no "anchor points" and those tests are hard to debug because the logs can't easily be parsed by a human.

ACKs for top commit:
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  instagibbs:
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  fanquake:
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Tree-SHA512: 08d962e85c4892c2a0c58feb5dc697c680a9d68e41a79417da6fcd415e0c5c735c4533a985cf225bb89deb5ca717d9bedf990657958079185804caa512b10f5a

Author: MarcoFalke

test/README.md

@@ -145,7 +145,7 @@ levels using the logger included in the test_framework, e.g.
   `test_framework.log` and no logs are output to the console.
 - when run directly, *all* logs are written to `test_framework.log` and INFO
   level and above are output to the console.
-- when run on Travis, no logs are output to the console. However, if a test
+- when run by [our CI (Continuous Integration)](/ci/README.md), no logs are output to the console. However, if a test
   fails, the `test_framework.log` and bitcoind `debug.log`s will all be dumped
   to the console to help troubleshooting.
 

test/functional/README.md

@@ -51,10 +51,13 @@ don't have test cases for.
 
 #### General test-writing advice
 
+- Instead of inline comments or no test documentation at all, log the comments to the test log, e.g.
+  `self.log.info('Create enough transactions to fill a block')`. Logs make the test code easier to read and the test
+  logic easier [to debug](/test/README.md#test-logging).
 - Set `self.num_nodes` to the minimum number of nodes necessary for the test.
   Having additional unrequired nodes adds to the execution time of the test as
   well as memory/CPU/disk requirements (which is important when running tests in
-  parallel or on Travis).
+  parallel).
 - Avoid stop-starting the nodes multiple times during the test if possible. A
   stop-start takes several seconds, so doing it several times blows up the
   runtime of the test.