gcc-4.8 on xenial fails unit test 4 (streaming)

[jeking3]

After introducing #40, this test started failing.
It did not fail on trusty with gcc 4.8 from the ubuntu-toolchain-r-test.
It fails on xenial with gcc 4.8.5 (part of xenial).
It does not fail on anything else. I suspect the xenial environment has an issue.
The file has a comment in it regarding leveraging somewhat undocumented behavior.

I have allowed this job to fail in CI for the time being.

Here's a build where it fails:

https://travis-ci.org/boostorg/dynamic_bitset/jobs/523097058

Here's the failure output:

gcc.link ../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4
testing.capture-output ../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4.run
====== BEGIN OUTPUT ======
../../boost/test/minimal.hpp(139): exception "std::ios_base::failure[abi:cxx11]: basic_ios::clear: iostream error" caught in function: 'int main(int, char**)'
**** Testing aborted.
**** 1 error detected
EXIT STATUS: 201
====== END OUTPUT ======
    LD_LIBRARY_PATH="/home/travis/build/boostorg/boost-root/bin.v2/libs/filesystem/build/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden:/home/travis/build/boostorg/boost-root/bin.v2/libs/system/build/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden:/usr/bin:/usr/lib:/usr/lib32:/usr/lib64:$LD_LIBRARY_PATH"
export LD_LIBRARY_PATH
    status=0
    if test $status -ne 0 ; then
        echo Skipping test execution due to testing.execute=off
        exit 0
    fi
     "../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4"   > "../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4.output" 2>&1 < /dev/null
    status=$?
    echo >> "../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4.output"
    echo EXIT STATUS: $status >> "../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4.output"
    if test $status -eq 0 ; then
        cp "../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4.output" "../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4.run"
    fi
    verbose=0
    if test $status -ne 0 ; then
        verbose=1
    fi
    if test $verbose -eq 1 ; then
        echo ====== BEGIN OUTPUT ======
        cat "../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4.output"
        echo ====== END OUTPUT ======
    fi
    exit $status
...failed testing.capture-output ../../bin.v2/libs/dynamic_bitset/test/dyn_bitset_unit_tests4.test/gcc-4.8/release/cxxstd-03-iso/threading-multi/visibility-hidden/dyn_bitset_unit_tests4.run...