Fix latent bug in test_gdb_complete_cmd_multiple

Fix latent bug in test_gdb_complete_cmd_multiple

A following patch will add the following to a testcase:

  test_gdb_completion_offers_commands "| "

And that tripped on a latent testsuite bug:

 (gdb) | PASS: gdb.base/shell.exp: tab complete "| "
 ^CQuit
 (gdb) complete |
 | !
 | +
 PASS: gdb.base/shell.exp: cmd complete "| "
 |  *** List may be truncated, max-completions reached. ***
 (gdb) FAIL: gdb.base/shell.exp: set max-completions 200
 set max-completions 200

The issue is that "|" ends up as part of a regexp, and "|" in regexps
has a special meaning...

Fix this with string_to_regexp.

gdb/testsuite/ChangeLog:
2019-07-03  Pedro Alves  <palves@redhat.com>

	* lib/completion-support.exp (test_gdb_complete_cmd_multiple): Use
	string_to_regexp.