[libcxxabi, libunwind] [test] Place output from tests under a 'test' subdir

Previously, all the output from the tests were placed directly in
the build directory. The tests produce a couple directories named
`__config_{exec,cache,src}__` which are easy to distinguish, and
the output from the individual tests were placed directly in a
directory named `Output`.

This is the same change as
736c6e246f5398331d83edd204a846cc967ad5c6, but for the libcxxabi
and libunwind test suites.

Differential Revision: https://reviews.llvm.org/D147628

GitOrigin-RevId: a2ef896a961c0180314acfde635f2b2f1e3625d4
diff --git a/test/configs/cmake-bridge.cfg.in b/test/configs/cmake-bridge.cfg.in
index 109602d..7860301 100644
--- a/test/configs/cmake-bridge.cfg.in
+++ b/test/configs/cmake-bridge.cfg.in
@@ -20,7 +20,7 @@
 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 = '@CMAKE_BINARY_DIR@'
+config.test_exec_root = os.path.join('@CMAKE_BINARY_DIR@', 'test')
 
 # Add a few features that are common to all the configurations
 if @LIBUNWIND_USES_ARM_EHABI@: