[string_view] Test failures on MacOS related to std::pmr

[Lastique]

See here, for example:

https://github.com/boostorg/core/actions/runs/7799837421/job/21271466543#step:6:15992

dyld[40635]: Symbol not found: __ZNSt3__13pmr20get_default_resourceEv
  Referenced from: <78D9D57E-6A40-31D9-B0ED-8D8C1EC12D2E> /Users/runner/work/core/core/boost-root/bin.v2/libs/core/test/sv_construct_test.test/clang-darwin-15/debug/cxxstd-17-iso/threading-multi/visibility-hidden/sv_construct_test
  Expected in:     <C39B7907-EABB-3019-AAB0-F6AEA01D5C1A> /usr/lib/libc++.1.dylib

EXIT STATUS: 134

This appeared only recently, I'm not sure what causes it. My guess is something has changed in Mac OS GHA images.

[pdimov]

This changed: actions/runner-images@87666d8

[pdimov]

Should be fixed with 2f092c6, although GHA seems to have switched back to an older macos-13 image so it's hard to tell.