[gdb/testsuite] Fix full buffer in gdb.rust/dwindex.exp

On ubuntu 18.04.5, I run into:
...
(gdb) mt print objfiles dwindex^M
^M
Object file build/gdb/testsuite/outputs/gdb.rust/dwindex/dwindex:  \
  Objfile at 0x55dab0b87a50, bfd at 0x55dab0b0cfa0, 1095 minsyms^M
^M
Psymtabs:^M
vendor/compiler_builtins/src/int/specialized_div_rem/mod.rs at 0x55dab0db0720^M
  ...
library/std/src/sys/unix/stdio.rs at 0x55dab0d96320^M
ERROR: internal buffer is full.
UNRESOLVED: gdb.rust/dwindex.exp: check if index present
...

Fix this by using -lbl in proc ensure_gdb_index.

Tested on x86_64-linux.

gdb/testsuite/lib/gdb.exp

@@ -7917,22 +7917,36 @@ proc add_gdb_index { program {style ""} } {
 # string (the default) means .gdb_index; "-dwarf-5" means .debug_names.
 
 proc ensure_gdb_index { binfile {style ""} } {
+    global decimal
+
     set testfile [file tail $binfile]
     set test "check if index present"
-    gdb_test_multiple "mt print objfiles ${testfile}" $test {
+    set has_index 0
-	-re -wrap "gdb_index.*" {
+    gdb_test_multiple "mt print objfiles ${testfile}" $test -lbl {
+	-re "\r\n\\.gdb_index: version ${decimal}(?=\r\n)" {
+	    set has_index 1
+	    gdb_test_lines "" $gdb_test_name ".*"
+	}
+	-re "\r\n\\.debug_names: exists(?=\r\n)" {
+	    set has_index 1
+	    gdb_test_lines "" $gdb_test_name ".*"
+	}
+	-re "\r\nPsymtabs:(?=\r\n)" {
+	    gdb_test_lines "" $gdb_test_name ".*"
+	}
+	-re -wrap "" {
+	    fail $gdb_test_name
+	}
+    }
+
+    if { $has_index } {
 	return 0
     }
-	-re -wrap "debug_names.*" {
+
-	    return 0
+    if { [add_gdb_index $binfile $style] == "1" } {
-	}
-	-re -wrap "Psymtabs.*" {
-	    if { [add_gdb_index $binfile $style] != "1" } {
-		return -1
-	    }
 	return 1
     }
-    }
+
     return -1
 }