[libcxx] [test] Fix MSVC warning C4800.

This was implicitly converting [1, 3] to bool, which triggers
an MSVC warning. The test should just pass `true`, which is
simpler, has the same behavior, and avoids the warning. (This
is a library test, not a compiler test, and the conversion happens
before calling `push_back`, so passing [1, 3] isn't interesting
in any way. This resembles a previous change to stop passing
`1 == 1` in the `vector<bool>` tests.)

llvm-svn: 346910