[gdb/testsuite] Fix prompt parsing in capture_command_output

[gdb/testsuite] Fix prompt parsing in capture_command_output

I noticed in capture_command_output that the output of a single command is
matched using two gdb_test_multiples:
- the first one matching the echoed command and skipping an optional prefix,
- the second one matching the output and the prompt.

This is error-prone, because the first gdb_test_multiple has implicit
clauses which may consume the prompt.

The problem is easy to spot with an example.  First consider:
...
set output [capture_command_output "print 1" "\\\$1 = "]
gdb_assert { [string equal $output "1"] }
...
for which we get:
...
PASS: [string equal $output "1"]
...

If we change the prefix string to a no-match, say "1 = ", and update the
output string match accordingly, we get instead:
...
FAIL: capture_command_output for print 1
FAIL: [string equal $output "\$1 = 1"]
...

The first FAIL is produced by the first gdb_test_multiple, consuming the prompt.

The second gdb_test_multiple then silently times out waiting for another prompt,
after which the second FAIL is produced.  Note that the timeout is silent
because the gdb_test_multiple is called with an empty message argument.

The second FAIL is because capture_command_output returns "", given that all
the command output was consumed by the first gdb_test_multiple.

Fix this by rewriting capture_command_output to use only a single
gdb_test_multiple.

Tested on x86_64-linux.