test: don't clobber RegExp.$_ on startup

Some tests expect it to be empty so clear it again after running a
regular expression against /proc/cpuinfo.

It didn't cause test failures on any of the CI machines, to the best of
my knowledge, because most of the time /proc/cpuinfo doesn't contain
the string it was looking for.

Fixes: #44840
PR-URL: #44864
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>

test/common/index.js

@@ -126,7 +126,9 @@ const isPi = (() => {
     // the contents of `/sys/firmware/devicetree/base/model` but that doesn't
     // work inside a container. Match the chipset model number instead.
     const cpuinfo = fs.readFileSync('/proc/cpuinfo', { encoding: 'utf8' });
-    return /^Hardware\s*:\s*(.*)$/im.exec(cpuinfo)?.[1] === 'BCM2835';
+    const ok = /^Hardware\s*:\s*(.*)$/im.exec(cpuinfo)?.[1] === 'BCM2835';
+    /^/.test('');  // Clear RegExp.$_, some tests expect it to be empty.
+    return ok;
   } catch {
     return false;
   }