[2.7] bpo-30764: test_subprocess uses SuppressCrashReport (#2405) (#2412)

[2.7] bpo-30764: test_subprocess uses SuppressCrashReport (#2405) (#2412)

* bpo-30764: Backport support.SuppressCrashReport

Backport test.support.SuppressCrashReport context-manager from
master. Drop the Windows implementation since it depends on
msvcrt.CrtSetReportMode() which isn't available on Python 2.7.

* bpo-30764: test_subprocess uses SuppressCrashReport (#2405)

bpo-30764, bpo-29335: test_child_terminated_in_stopped_state() of
test_subprocess now uses support.SuppressCrashReport() to prevent the
creation of a core dump on FreeBSD.
(cherry picked from commit cdee3f14f7f4c995e7eedb0bf6a67e260c739f7d)