[runtimes] Avoid cluttering the top-level build directory with test artifacts (#112717)

Instead of placing artifacts for testing the runtimes at <build>/test,
place those artifacts at <build>/<project>/test. This prevents
cluttering the build directory with the runtimes' test artifacts for
everyone else.

As a drive-by, remove LIBCXX_BINARY_INCLUDE_DIR which wasn't used
anymore.

NOKEYCHECK=True
GitOrigin-RevId: 8c77f4c5087ac5a8e5dc08e472cf06897689a68b

Author: ldionne

test/configs/cmake-bridge.cfg.in

@@ -20,7 +20,7 @@ config.name = os.path.basename('@LIBUNWIND_TEST_CONFIG@')
 config.test_source_root = os.path.join('@LIBUNWIND_SOURCE_DIR@', 'test')
 config.test_format = libcxx.test.format.CxxStandardLibraryTest()
 config.recursiveExpansionLimit = 10
-config.test_exec_root = os.path.join('@CMAKE_BINARY_DIR@', 'test')
+config.test_exec_root = os.path.join('@LIBUNWIND_BINARY_DIR@', 'test')
 
 # Add a few features that are common to all the configurations
 if @LIBUNWIND_USES_ARM_EHABI@: