test: add buffer transcode test

This test is for argument validation in transcode.

feedback from approvers; full regex error msg & new line

Author: Johnny Reading

test/parallel/test-icu-transcode.js

@@ -38,11 +38,17 @@ for (const test in tests) {
                      utf8_to_ucs2.toString('ucs2'));
 }
 
+assert.throws(
+  () => buffer.transcode(null, 'utf8', 'ascii'),
+  /^TypeError: "source" argument must be a Buffer$/
+);
+
 assert.throws(
   () => buffer.transcode(Buffer.from('a'), 'b', 'utf8'),
-  /Unable to transcode Buffer \[U_ILLEGAL_ARGUMENT_ERROR\]/
+  /^Error: Unable to transcode Buffer \[U_ILLEGAL_ARGUMENT_ERROR\]/
 );
+
 assert.throws(
   () => buffer.transcode(Buffer.from('a'), 'uf8', 'b'),
-  /Unable to transcode Buffer \[U_ILLEGAL_ARGUMENT_ERROR\]/
+  /^Error: Unable to transcode Buffer \[U_ILLEGAL_ARGUMENT_ERROR\]$/
 );