[gdb/testsuite] Fix gdb.guile/scm-breakpoint.exp with guile 3.0

When running test-case gdb.guile/scm-breakpoint.exp on openSUSE Tumbleweed
with guile 3.0, I run into:
...
(gdb) guile (define cp (make-breakpoint "syscall" #:type BP_CATCHPOINT))^M
ERROR: In procedure make-breakpoint:^M
In procedure gdbscm_make_breakpoint: unsupported breakpoint type in \
  position 3: "BP_CATCHPOINT"^M
Error while executing Scheme code.^M
(gdb) FAIL: gdb.guile/scm-breakpoint.exp: test_catchpoints: \
  create a catchpoint via the api
...

The same test passes on openSUSE Leap 15.2 with guile 2.0, where the second
line of the error message starts with the same prefix as the first:
...
ERROR: In procedure gdbscm_make_breakpoint: unsupported breakpoint type in \
  position 3: "BP_CATCHPOINT"^M
...

I observe the same difference in many other tests, f.i.:
...
(gdb) gu (print (value-add i '()))^M
ERROR: In procedure value-add:^M
In procedure gdbscm_value_add: Wrong type argument in position 2: ()^M
Error while executing Scheme code.^M
(gdb) PASS: gdb.guile/scm-math.exp: catch error in guile type conversion
...
but it doesn't cause FAILs anywhere else.

Fix this by updating the regexp to make the "ERROR: " prefix optional.

Tested on x86_64-linux, with both guile 2.0 and 3.0.

gdb/testsuite/ChangeLog:

2021-07-07  Tom de Vries  <tdevries@suse.de>

	* gdb.guile/scm-breakpoint.exp: Make additional "ERROR: " prefix in
	exception printing optional.

1 files changed, 3 insertions, 3 deletions

diff --git a/gdb/testsuite/gdb.guile/scm-breakpoint.exp b/gdb/testsuite/gdb.guile/scm-breakpoint.exp
index c603af7..be898ca 100644
--- a/gdb/testsuite/gdb.guile/scm-breakpoint.exp
+++ b/gdb/testsuite/gdb.guile/scm-breakpoint.exp
@@ -260,10 +260,10 @@ proc_with_prefix test_watchpoints { } {
 	"test watchpoint write"
 
     gdb_test "guile (define wp2 (make-breakpoint \"result\" #:wp-class WP_WRITE #:type 999))" \
-	"ERROR: In procedure gdbscm_make_breakpoint: Out of range: invalid breakpoint type in position 5: 999\r\n.*" \
+	"(ERROR: )?In procedure gdbscm_make_breakpoint: Out of range: invalid breakpoint type in position 5: 999\r\n.*" \
 	"create a breakpoint with an invalid type number"
     gdb_test "guile (define wp2 (make-breakpoint \"result\" #:wp-class WP_WRITE #:type BP_NONE))" \
-	"ERROR: In procedure gdbscm_make_breakpoint: unsupported breakpoint type in position 5: \"BP_NONE\"\r\n.*" \
+	"(ERROR: )?In procedure gdbscm_make_breakpoint: unsupported breakpoint type in position 5: \"BP_NONE\"\r\n.*" \
 	"create a breakpoint with an unsupported type"
 }
 
@@ -533,7 +533,7 @@ proc_with_prefix test_catchpoints {} {
     # Try to create a catchpoint, currently this isn't supported via
     # the guile api.
     gdb_test "guile (define cp (make-breakpoint \"syscall\" #:type BP_CATCHPOINT))" \
-	"ERROR: In procedure gdbscm_make_breakpoint: unsupported breakpoint type in position 3: \"BP_CATCHPOINT\"\r\n.*" \
+	"(ERROR: )?In procedure gdbscm_make_breakpoint: unsupported breakpoint type in position 3: \"BP_CATCHPOINT\"\r\n.*" \
 	"create a catchpoint via the api"
 
     # Setup a catchpoint.