Mark test_gen_exports's test_process as run_slow

[A5rocks]

Locally each parametrize of this takes 2 seconds. Given there's 3 of them, that's more than 20% of the runtime of pytest locally!

For reference:

================================================ slowest 10 durations =================================================
2.19s call     src/trio/_tests/tools/test_gen_exports.py::test_process[from collections import Counter\n]
2.13s call     src/trio/_tests/tools/test_gen_exports.py::test_process[from collections import Counter\nimport os\n]
2.04s call     src/trio/_tests/test_highlevel_open_tcp_listeners.py::test_open_tcp_listeners_ipv6_v6only
2.03s call     src/trio/_tests/test_socket.py::test_address_in_socket_error
2.02s call     src/trio/_tests/test_socket.py::test_SocketType_connect_paths
2.00s call     src/trio/_tests/tools/test_gen_exports.py::test_process[from typing import TYPE_CHECKING\nif TYPE_CHECKING:\n    from collections import Counter\n]
1.52s call     src/trio/_tests/test_socket.py::test_many_sockets
0.70s call     src/trio/_tests/test_exports.py::test_static_tool_sees_all_symbols[jedi-trio]
0.56s call     src/trio/_tests/tools/test_gen_exports.py::test_lint_failure
0.51s call     src/trio/_tests/test_util.py::test_coroutine_or_error
===================================== 769 passed, 77 skipped, 2 xfailed in 28.59s =====================================

[A5rocks]

More generally anything above the 1 second mark is kind of suspect. Many of the network tests simply don't need that many runs too -- for instance test_many_sockets is 10x what it could be. (based on comments in the test case)