As a by-product, this fixes the known problems with abc-cmdline.py when run
out-of-tree, and allows the test to be taken back into regular execution in the
test suite.
The abc-cmdline.py test illustrates how tests can check their expected stderr
traces.
Refer also to the OutputChecker class in qa/rpc-tests/test_framework/outputchecker.py.
If the test framework is unable to locate a bitcoind binary, a test will now output
an error message:
Unable to locate bitcoind for this test.
It is still possible to set SRCDIR to the location of the folder in which the src/
folder resides, to override in special cases.