Merge bitcoin/bitcoin#30859: doc: cmake: prepend "build" to functional/test_runner.py

e64b2f1a16 doc: cmake: prepend and explain "build/" where needed (Larry Ruane)

Pull request description:

  This is a small follow-on to #30741, prepend `build/` to the path for `test_runner.py`.

ACKs for top commit:
  jonatack:
    ACK e64b2f1a16
  maflcko:
    lgtm ACK e64b2f1a16
  tdb3:
    re ACK e64b2f1a16

Tree-SHA512: 80943d4f342987bf060adacb1c7db2e9ff8de5a6da592846ba23f230281d3a5b306162c4c86e61739a29323eaa4abf09f69f41302996d5809f448e5788a74a87

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`
+(assuming `build` is your 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 +10,8 @@ that file and modify to fit your needs.
 
 #### Coverage
 
-Running `test/functional/test_runner.py` with the `--coverage` argument tracks which RPCs are
+Assuming the build directory is `build`,
+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.