Merge #11219: [docs] explain how to recompile a modified unit test

478d4fb79 [docs] explain how to recompile only what bitcoind tests need (Sjors Provoost)

Pull request description:

  It was not obvious to me to run `make` inside the test directory, especially because `make src/test` and `make src/test/test_bitcoin` result in `make: Nothing to be done for ...`.

Tree-SHA512: 5fe66c45c50af42d4fed42e3008b1dc4de7ea448f5265a34f4b2f355aa4a48a8187918a49fc9f82e8dd9706bc72c59d0fd67d86057fd816eb317832e46ada7ba

src/test/README.md

@@ -5,7 +5,10 @@ and tests weren't explicitly disabled.
 
 After configuring, they can be run with `make check`.
 
-To run the bitcoind tests manually, launch `src/test/test_bitcoin`.
+To run the bitcoind tests manually, launch `src/test/test_bitcoin`. To recompile
+after a test file was modified, run `make` and then run the test again. If you
+modify a non-test file, use `make -C src/test` to recompile only what's needed
+to run the bitcoind tests.
 
 To add more bitcoind tests, add `BOOST_AUTO_TEST_CASE` functions to the existing
 .cpp files in the `test/` directory or add new .cpp files that