7a0c2242890549b6dddac4cc6f1840a528469f2a Suppress output in test_bitcoin for expected errors (Gert-Jaap Glasbergen)
Pull request description:
Closes#15944
This adds two methods to noui, that allows temporarily suppressing (and then resuming) the output from `noui`. For situations where errors are expected, it's confusing for the test binary to output an error and then conclude with `No errors detected`.
It also uses this supress/reconnect in the tests that currently produce verbose errors when running `test_bitcoin`.
Output of `test_bitcoin` on current master:
```
gertjaap@gjdesktop:~/src/bitcoin$ src/test/test_bitcoin
Running 351 test cases...
Error: Specified -walletdir "/tmp/test_common_Bitcoin Core/1561389554_943311758/tempdir/path_does_not_exist" does not exist
Error: Specified -walletdir "/tmp/test_common_Bitcoin Core/1561389554_643733972/tempdir/not_a_directory.dat" is not a directory
Error: Specified -walletdir "wallets" is a relative path
*** No errors detected
```
Output after this code is merged:
```
gertjaap@gjdesktop:~/src/bitcoin$ src/test/test_bitcoin
Running 351 test cases...
*** No errors detected
```
ACKs for top commit:
l2a5b1:
ACK 7a0c224 - tested and reviewed.
laanwj:
ACK 7a0c2242890549b6dddac4cc6f1840a528469f2a
Tree-SHA512: c7881f7a431a065329360ffa9937ce4742694c646c90c019d3aff95dfd7fccbdcda9116c5762feb6dfd1108d14f9fb386e203b173c4bde9093afb2b8c977d13d