tests: fix path in instructions

Reported by notmike

tests/README.md

@@ -18,7 +18,7 @@ Tests are located in `tests/core_tests/`, and follow a straightforward naming co
 To run only Monero's core tests (after building):
 
 ```
-cd build/debug/tests/core
+cd build/debug/tests/core_tests
 ctest
 ```
 
@@ -103,6 +103,8 @@ cd build/debug/tests/performance_tests
 ./performance_tests
 ```
 
+The path may be build/Linux/master/debug (adapt as necessary for your platform).
+
 If the `performance_tests` binary does not exist, try running `make` in the `build/debug/tests/performance_tests` directory.
 
 To run the same tests on a release build, replace `debug` with `release`.