threads.policy_converting_constructor_from_other_policy test hanging in nightly Pthreads builds with gcc

[ndellingwood]

Following merge of PR #3491, nightly Pthreads builds with gcc started timing out in the policy_converting_constructor_from_other_policy unit test

Sample reproducer instructions (gcc/8.3.0):

module load sems-env sems-cmake/3.12.2 sems-gcc/8.3.0

# Via generate_makefile
$KOKKOS_PATH/generate_makefile.bash --with-devices=Pthread,Serial --arch=SNB,Volta70 --compiler=g++ --cxxflags="-O3 -Wall -Wunused-parameter -Wshadow -pedantic -Werror -Wsign-compare -Wtype-limits -Wignored-qualifiers -Wempty-body -Wclobbered -Wuninitialized " --cxxstandard="14" --kokkos-path=$KOKKOS_PATH --no-examples

# Corresponding cmake line
cmake -DCMAKE_CXX_COMPILER=`which g++` -DCMAKE_CXX_FLAGS="-O3 -Wall -Wunused-parameter -Wshadow -pedantic -Werror -Wsign-compare -Wtype-limits -Wignored-qualifiers -Wempty-body -Wclobbered -Wuninitialized" -DKokkos_ENABLE_SERIAL=ON -DKokkos_ENABLE_PTHREAD=ON -DKokkos_ARCH_VOLTA70=ON -DKokkos_ARCH_SNB=ON -DKokkos_ENABLE_TESTS=ON -DKokkos_ENABLE_EXAMPLES=OFF -DCMAKE_VERBOSE_MAKEFILE=ON -DCMAKE_CXX_EXTENSIONS=OFF -DCMAKE_CXX_STANDARD=14 -DCMAKE_BUILD_TYPE=RELEASE $KOKKOS_PATH

cd core/unit_tests

./KokkosCore_UnitTest_Threads --gtest_filter=threads.policy_converting_constructor_from_other_policy

[ndellingwood]

Addressed by PR #3510