test: increased code coverage for slowCases

Added test coverage for 4 un-covered if statements in slowCases

PR-URL: #23592
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

test/parallel/test-internal-util-normalizeencoding.js

@@ -18,6 +18,7 @@ const tests = [
   ['utF-8', 'utf8'],
   ['ucs2', 'utf16le'],
   ['UCS2', 'utf16le'],
+  ['UcS2', 'utf16le'],
   ['ucs-2', 'utf16le'],
   ['UCS-2', 'utf16le'],
   ['UcS-2', 'utf16le'],
@@ -31,8 +32,11 @@ const tests = [
   ['LaTiN1', 'latin1'],
   ['base64', 'base64'],
   ['BASE64', 'base64'],
+  ['Base64', 'base64'],
   ['hex', 'hex'],
   ['HEX', 'hex'],
+  ['ASCII', 'ascii'],
+  ['AsCii', 'ascii'],
   ['foo', undefined],
   [1, undefined],
   [false, undefined],