[libcxx] [test] Place output from newconfig tests under a 'test' subdir

[mstorsjo]

Summary

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, but
the output from the individual tests are placed in subdirectories
named std or libcxx. Especially the build output from libcxx
tests ends up conflated in the same directories as are used for
building libcxx with cmake.

libcxx/test/configs/cmake-bridge.cfg.in

 site.addsitedir(os.path.join('@LIBCXX_SOURCE_DIR@', 'utils'))
 import libcxx.test.format
 
 # Basic configuration of the test suite
 config.name = os.path.basename('@LIBCXX_TEST_CONFIG@')
 config.test_source_root = os.path.join('@LIBCXX_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 substitutions for bootstrapping the test suite configuration
 config.substitutions.append(('%{cxx}', shlex.quote('@CMAKE_CXX_COMPILER@')))
 config.substitutions.append(('%{libcxx}', '@LIBCXX_SOURCE_DIR@'))
 config.substitutions.append(('%{install}', '@CMAKE_BINARY_DIR@'))
 config.substitutions.append(('%{include}', '%{install}/@LIBCXX_INSTALL_INCLUDE_DIR@'))
 config.substitutions.append(('%{target-include}', '%{install}/@LIBCXX_INSTALL_INCLUDE_TARGET_DIR@'))
 config.substitutions.append(('%{lib}', '%{install}/@LIBCXX_INSTALL_LIBRARY_DIR@'))
 config.substitutions.append(('%{executor}', '@LIBCXX_EXECUTOR@'))