cleaning up termios constants (#13138)

stdlib/@tests/stubtest_allowlists/darwin.txt

@@ -19,9 +19,6 @@ select.POLLMSG   # system dependent
 (fcntl.F_SETLEASE)?
 (_gdbm)?
 
-# Platform differences that cannot be captured by the type system
-(termios.[A-Z0-9_]+)?
-
 # ==========
 # Allowlist entries that cannot or should not be fixed
 # ==========

stdlib/@tests/stubtest_allowlists/linux.txt

@@ -25,7 +25,6 @@ multiprocessing.popen_spawn_win32
 
 # Platform differences that cannot be captured by the type system
 fcntl.I_[A-Z0-9_]+
-(termios.[A-Z0-9_]+)?
 
 # Some of these exist on non-windows, but they are useless and this is not intended
 stat.FILE_ATTRIBUTE_[A-Z_]+
@@ -66,3 +65,7 @@ _?socket.SOL_HCI
 _?socket.SOL_IPX
 _?socket.SOL_NETROM
 _?socket.SOL_ROSE
+
+# This is available on Linux, but it's documented as for kernel debugging and
+# not present on GitHub Actions runners.
+termios.TIOCTTYGSTRUCT