[gdb/testsuite] Fix gdb.python/py-format-address.exp on arm

When running test-case gdb.python/py-format-address.exp on arm-linux, I get:
...
(gdb) python print("Got: " + gdb.format_address(0x103dd))^M
Got: 0x103dd <main at py-format-address.c:30>^M
(gdb) FAIL: $exp: symbol_filename=on: gdb.format_address, \
result should have an offset
...

What is expected here is:
...
Got: 0x103dd <main+1 at py-format-address.c:30>^M
...

Main starts at main_addr:
...
(gdb) print /x &main^M
$1 = 0x103dc^M
...
and we obtained next_addr 0x103dd by adding 1 to it:
...
set next_addr [format 0x%x [expr $main_addr + 1]]
...

Adding 1 to $main_addr results in an address for a thumb function starting at
address 0x103dc, which is incorrect because main is an arm function (because
I'm running with target board unix/-marm).

At some point during the call to format_addr, arm_addr_bits_remove removes
the thumb bit, which causes the +1 offset to be dropped, causing the FAIL.

Fix this by using the address of the breakpoint on main, provided it's not at
the very start of main.

Tested on arm-linux.

PR testsuite/31452
Bug: https://www.sourceware.org/bugzilla/show_bug.cgi?id=31452

gdb/testsuite/gdb.python/py-format-address.exp

@@ -40,7 +40,23 @@ if ![runto_main] {
 # for the program space and architecture (these will be selected based
 # on the current inferior).
 set main_addr [get_hexadecimal_valueof "&main" "UNKNOWN"]
-set next_addr [format 0x%x [expr $main_addr + 1]]
+require {!string equal $main_addr {UNKNOWN}}
+
+set next_addr "UNKNOWN"
+gdb_test_multiple "info break 1" "" {
+    -re -wrap " y +($hex) +in .*" {
+	set next_addr $expect_out(1,string)
+	set next_addr [regsub {^0x0+} $next_addr "0x"]
+	pass $gdb_test_name
+    }
+}
+if { $next_addr == "UNKNOWN" || $next_addr == $main_addr } {
+    set next_addr [format 0x%x [expr $main_addr + 1]]
+}
+
+verbose -log "main_addr: $main_addr"
+verbose -log "next_addr: $next_addr"
+
 
 foreach_with_prefix symbol_filename { on off } {
     gdb_test_no_output "set print symbol-filename ${symbol_filename}"
@@ -56,7 +72,7 @@ foreach_with_prefix symbol_filename { on off } {
 	"gdb.format_address, result should have no offset"
 
     gdb_test "python print(\"Got: \" + gdb.format_address($next_addr))" \
-	"Got: $next_addr <main\\+1${filename_pattern}>" \
+	"Got: $next_addr <main\\+$decimal${filename_pattern}>" \
 	"gdb.format_address, result should have an offset"
 }