[test] Allow libc++ namespaces besides `__1`

The libc++ data formatter for `std::shared_ptr` allows any namespace, but the test asserts that it must be the default `__1` namespace. Relax the regex to allow anything that looks like `__.*` (although we use `__[^:]*` so we don't match arbitrarily long text).

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D129898