[BE] install flaggems in CI and unskip tests

[jiannanWang]

Install flaggems in CI and unskip three tests.

Before:

test/test_backends.py::TestFlagGemsBackend::test_flag_gems_backend_initialization SKIPPED [  7%]
test/test_backends.py::TestFlagGemsBackend::test_flag_gems_backend_contains_op SKIPPED [  8%]
test/test_backends.py::TestFlagGemsBackend::test_flag_gems_backend_getitem SKIPPED [  9%]

After:

test/test_backends.py::TestFlagGemsBackend::test_flag_gems_backend_initialization PASSED [  7%]
test/test_backends.py::TestFlagGemsBackend::test_flag_gems_backend_contains_op PASSED [  8%]
test/test_backends.py::TestFlagGemsBackend::test_flag_gems_backend_getitem PASSED [  9%]