test: change tmp directories prefix

`.tmp` prefix allows easier exclusion

PR-URL: nodejs/node#16372
Refs: nodejs/node#16010
Refs: nodejs/node#16278
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

.eslintignore

@@ -2,7 +2,6 @@ lib/internal/v8_prof_polyfill.js
 lib/punycode.js
 test/addons/??_*
 test/fixtures
-test/tmp*
 tools/eslint
 tools/icu
 node_modules

test/common/index.js

@@ -38,7 +38,9 @@ const noop = () => {};
 
 exports.fixturesDir = fixturesDir;
 
-exports.tmpDirName = 'tmp';
+// Using a `.` prefixed name, which is the convention for "hidden" on POSIX,
+// gets tools to ignore it by default or by simple rules, especially eslint.
+exports.tmpDirName = '.tmp';
 // PORT should match the definition in test/testpy/__init__.py.
 exports.PORT = +process.env.NODE_COMMON_PORT || 12346;
 exports.isWindows = process.platform === 'win32';