[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.

gdb/testsuite/gdb.testsuite/capture-command-output.exp

@@ -0,0 +1,38 @@
+# Copyright 2022 Free Software Foundation, Inc.
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# The purpose of this test-case is to test the capture_command_output proc.
+
+clean_restart
+
+# Check output with no prefix.
+
+with_test_prefix no-prefix {
+    set output [capture_command_output "print 1" ""]
+    gdb_assert { [string equal $output "\$1 = 1"] }
+}
+
+# Check output with matching prefix.
+
+with_test_prefix matching-prefix {
+    set output [capture_command_output "print 1" "\\\$2 = "]
+    gdb_assert { [string equal $output "1"] }
+}
+
+# Check output with non-matching prefix.
+
+with_test_prefix non-matching-prefix {
+    set output [capture_command_output "print 1" "3 = "]
+    gdb_assert { [string equal $output "\$3 = 1"] }
+}

gdb/testsuite/lib/gdb.exp

@@ -7888,27 +7888,10 @@ proc capture_command_output { command prefix } {
     global gdb_prompt
     global expect_out
 
-    set code {
-	-re "^[string_to_regexp ${command}]\r\n" {
-	    if { $prefix != "" } {
-		exp_continue
-	    }
-	}
-    }
-
-    if { $prefix != "" } {
-	append code {
-	    -re "^${prefix}" {
-		# Nothing, we just move onto the next gdb_test_multiple
-		# call, which actually collects the command output.
-	    }
-	}
-    }
-
-    gdb_test_multiple "$command" "capture_command_output for $command" $code
+    set test "capture_command_output for $command"
 
     set output_string ""
-    gdb_test_multiple "" "" {
+    gdb_test_multiple $command $test {
 	-re "^(\[^\r\n\]+\r\n)" {
 	    if { ![string equal $output_string ""] } {
 		set output_string [join [list $output_string $expect_out(1,string)] ""]
@@ -7922,7 +7905,18 @@ proc capture_command_output { command prefix } {
 	}
     }
 
+    # Strip the command.
+    set command_re [string_to_regexp ${command}]
+    set output_string [regsub ^$command_re\r\n $output_string ""]
+
+    # Strip the prefix.
+    if { $prefix != "" } {
+	set output_string [regsub ^$prefix $output_string ""]
+    }
+
+    # Strip a trailing newline.
     set output_string [regsub "\r\n$" $output_string ""]
+
     return $output_string
 }