Move get_signal_name() to test.support (#121251)

* Move get_signal_name() from test.libregrtest to test.support.
* Use get_signal_name() in support.script_helper.
* support.script_helper now decodes stdout and stderr from UTF-8,
  instead of ASCII, if a command failed.

Lib/test/libregrtest/utils.py

@@ -685,35 +685,6 @@ def cleanup_temp_dir(tmp_dir: StrPath):
             print("Remove file: %s" % name)
             os_helper.unlink(name)
 
-WINDOWS_STATUS = {
-    0xC0000005: "STATUS_ACCESS_VIOLATION",
-    0xC00000FD: "STATUS_STACK_OVERFLOW",
-    0xC000013A: "STATUS_CONTROL_C_EXIT",
-}
-
-def get_signal_name(exitcode):
-    if exitcode < 0:
-        signum = -exitcode
-        try:
-            return signal.Signals(signum).name
-        except ValueError:
-            pass
-
-    # Shell exit code (ex: WASI build)
-    if 128 < exitcode < 256:
-        signum = exitcode - 128
-        try:
-            return signal.Signals(signum).name
-        except ValueError:
-            pass
-
-    try:
-        return WINDOWS_STATUS[exitcode]
-    except KeyError:
-        pass
-
-    return None
-
 
 ILLEGAL_XML_CHARS_RE = re.compile(
     '['