doc: cmake: prepend "build" to functional/test_runner.py

Also improve the error message if someone runs the functional tests
the old way (outside of the build directory).

README.md

@@ -53,7 +53,8 @@ and extending unit tests can be found in [/src/test/README.md](/src/test/README.
 
 There are also [regression and integration tests](/test), written
 in Python.
-These tests can be run (if the [test dependencies](/test) are installed) with: `test/functional/test_runner.py`
+These tests can be run (if the [test dependencies](/test) are installed) with: `build/test/functional/test_runner.py`.
+Do not run the (identical) program `test/functional/test_runner.py` (without the build directory).
 
 The CI (Continuous Integration) systems make sure that every pull request is built for Windows, Linux, and macOS,
 and that unit/sanity tests are run automatically.

test/functional/README.md

@@ -10,7 +10,7 @@ that file and modify to fit your needs.
 
 #### Coverage
 
-Running `test/functional/test_runner.py` with the `--coverage` argument tracks which RPCs are
+Running `build/test/functional/test_runner.py` with the `--coverage` argument tracks which RPCs are
 called by the tests and prints a report of uncovered RPCs in the summary. This
 can be used (along with the `--extended` argument) to find out which RPCs we
 don't have test cases for.

test/functional/test_framework/test_framework.py

@@ -206,6 +206,8 @@ def parse_args(self, test_file):
         self.options.timeout_factor = self.options.timeout_factor or (4 if self.options.valgrind else 1)
         self.options.previous_releases_path = previous_releases_path
 
+        if not os.path.isfile(self.options.configfile):
+            raise Exception(f"config.ini file {self.options.configfile} not found, be sure to run the this script from within the build directory")
         config = configparser.ConfigParser()
         config.read_file(open(self.options.configfile))
         self.config = config

test/functional/test_runner.py

@@ -462,6 +462,8 @@ def main():
     # Read config generated by configure.
     config = configparser.ConfigParser()
     configfile = os.path.abspath(os.path.dirname(__file__)) + "/../config.ini"
+    if not os.path.isfile(configfile):
+        raise Exception(f"config.ini file {configfile} not found, be sure to run the this script from within the build directory")
     config.read_file(open(configfile, encoding="utf8"))
 
     passon_args.append("--configfile=%s" % configfile)