gdb/testsuite: fix gdb.ada/complete.exp timeout in READ1 mode

gdb/testsuite: fix gdb.ada/complete.exp timeout in READ1 mode

While reviewing another patch I spotted a timeout in
gdb.ada/complete.exp when testing in READ1 mode, e.g.:

  $ make check-read1 TESTS="gdb.ada/complete.exp"
  ...
  FAIL: gdb.ada/complete.exp: complete break ada (timeout)
  ...

The problem is an attempt to match the entire output from GDB within a
single gdb_test_multiple pattern, for a completion command that
returns a large number of completions.

This commit changes the gdb_test_multiple to process the output line
by line.  I don't use the gdb_test_multiple -lbl option, as I've
always found that option backward -- it checks for the \r\n at the
start of each line rather than the end, I think it's much clearer to
use '^' at the start of each pattern, and '\r\n' at the end, so that's
what I've done here.

.... Or I would, if this test didn't already define $eol as the end of
line regexp ... except that $eol was set to '[\r\n]*', which isn't
that helpful, so I've updated $eol to be just '\r\n' the actual end of
line regexp.

And now, the test passes without a timeout when using READ1.

There should be no change in what is tested after this commit.

Approved-By: Tom Tromey <tom@tromey.com>