linux ttyname{_r}: Add tests

Add a new tst-ttyname test that includes several named sub-testcases.

This patch is ordered after the patches with the fixes that it tests for (to
avoid breaking `git bisect`), but for reference, here's how each relevant change
so far affected the testcases in this commit, starting with
15e9a4f3:

  |                                 | before  |         | make checks | don't |
  |                                 | 15e9a4f3 | 15e9a4f3 | consistent  | bail  |
  |---------------------------------+---------+---------+-------------+-------|
  | basic smoketest                 | PASS    | PASS    | PASS        | PASS  |
  | no conflict, no match           | PASS[1] | PASS    | PASS        | PASS  |
  | no conflict, console            | PASS    | FAIL!   | FAIL        | PASS! |
  | conflict, no match              | FAIL    | PASS!   | PASS        | PASS  |
  | conflict, console               | FAIL    | FAIL    | FAIL        | PASS! |
  | with readlink target            | PASS    | PASS    | PASS        | PASS  |
  | with readlink trap; fallback    | FAIL    | FAIL    | FAIL        | PASS! |
  | with readlink trap; no fallback | FAIL    | PASS!   | PASS        | PASS  |
  | with search-path trap           | FAIL    | FAIL    | PASS!       | PASS  |
  |---------------------------------+---------+---------+-------------+-------|
  |                                 | 4/9     | 5/9     | 6/9         | 9/9   |

  [1]: 15e9a4f3 introduced a semantic that, under certain failure
       conditions, ttyname sets errno=ENODEV, where previously it didn't
       set errno; it's not quite fair to hold "before 15e9a4f3" ttyname to
       those new semantics.  This testcase actually fails, but would have
       passed if we tested for the old the semantics.

Each of the failing tests before 15e9a4f3 are all essentially the same bug: that
it returns a PTY slave with the correct minor device number, but from the wrong
devpts filesystem instance.

15e9a4f3 sought to fix this, but missed several of the cases that can cause this
to happen, and also broke the case where both the erroneous PTY and the correct
PTY exist.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>