[gdb/testsuite] Fix gdb.base/backtrace-through-cu-nodebug.exp without python support

With a gdb build without python support, and test-case
gdb.base/backtrace-through-cu-nodebug.exp I run into:
...
(gdb) bt^M
Required frame unwinder may have been disabled, \
  see 'maint info frame-unwinders'^M
(gdb) FAIL: $exp: verify unwind fail without CFI
...

With a gdb build with python support we have instead:
...
(gdb) bt^M
Python Exception <class 'gdb.error'>: \
  Required frame unwinder may have been disabled, \
  see 'maint info frame-unwinders'^M
(gdb) PASS: $exp: verify unwind fail without CFI
...
but if I change the "bt" into "bt -no-filters" I get the same FAIL and
corresponding output.

So there are two scenarios here.

In the first:
- the bt command is called
- frame #0 is printed
- trying to get the next frame fails and an error is thrown, aborting the
  backtrace
- the error is caught and printed

In the second:
- the bt command is called
- the frame filter is applied
- doing so triggers the same error, which is caught and printed by
  gdbpy_apply_frame_filter, returning EXT_LANG_BT_NO_FILTERS
- frame #0 is printed
- getting the next frame fails, and the backtrace stops

It seems worthwhile to exercise both scenarios if possible, so add a
"bt -no-filters" test.

Fix the FAIL by updating the regexp to allow both scenarios.

Tested on aarch64-linux.

Reviewed-By: Keith Seitz <[email protected]>

Author: vries

gdb/testsuite/gdb.base/backtrace-through-cu-nodebug.exp

@@ -73,10 +73,32 @@ if {[gdb_compile "${srcdir}/${subdir}/${srcfile2}" \
 }
 
 if { [prepare_test false] } {
-    gdb_test "bt" \
+    set re_msg \
+	[string_list_to_regexp \
+	     "Required frame unwinder may have been disabled," \
+	     " see 'maint info frame-unwinders'"]
+    set hs {[^\r\n]}
+    set re_bt_line "#0\\s+[string_to_regexp {callback ()}] $hs+"
+    set re_bt_no_filters \
 	[multi_line \
-	     "\[^\r\n\]+Required frame unwinder may have been disabled, \[^\r\n\]+" \
-	     "#0\\s+callback \\(\\) \[^\r\n\]+"] \
+	     $re_bt_line \
+	     $re_msg]
+    gdb_test "bt -no-filters" \
+	$re_bt_no_filters \
+	"verify no-filters unwind fail without CFI"
+
+    # Flush frame cache to retrigger the message.
+    gdb_test "maint flush register-cache" \
+	[string_to_regexp "Register cache flushed."]
+
+    # This output may occur when we run into the message while applying the
+    # frame filters.
+    set re_bt \
+	[multi_line \
+	     $hs+$re_msg \
+	     $re_bt_line]
+    gdb_test "bt" \
+	"($re_bt|$re_bt_no_filters)" \
 	"verify unwind fail without CFI"
 }