test: add promise API test for appendFile()

Apply the second of five test cases in test-fs-append-fil to the
promise-based API in addition to the callback-based API.
PR-URL: #20842
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: Trott

test/parallel/test-fs-append-file.js

@@ -70,22 +70,35 @@ const throwNextTick = (e) => { process.nextTick(() => { throw e; }); };
 .catch(throwNextTick);
 }
 
-// test that appends data to a non empty file
-constfilename2=join(tmpdir.path,'append2.txt');
-fs.writeFileSync(filename2,currentFileData);
+// test that appends data to a non-empty file (callback API)
+{
+constfilename=join(tmpdir.path,'append-non-empty.txt');
+fs.writeFileSync(filename,currentFileData);
 
-fs.appendFile(filename2,s,function(e){
-assert.ifError(e);
+fs.appendFile(filename,s,common.mustCall(function(e){
+assert.ifError(e);
 
-ncallbacks++;
+fs.readFile(filename,common.mustCall(function(e,buffer){
+assert.ifError(e);
+assert.strictEqual(Buffer.byteLength(s)+currentFileData.length,
+buffer.length);
+}));
+}));
+}
 
-fs.readFile(filename2,function(e,buffer){
-assert.ifError(e);
-ncallbacks++;
-assert.strictEqual(Buffer.byteLength(s)+currentFileData.length,
-buffer.length);
-});
-});
+// test that appends data to a non-empty file (promise API)
+{
+constfilename=join(tmpdir.path,'append-non-empty-promise.txt');
+fs.writeFileSync(filename,currentFileData);
+
+fs.promises.appendFile(filename,s)
+.then(common.mustCall(()=>fs.promises.readFile(filename)))
+.then((buffer)=>{
+assert.strictEqual(Buffer.byteLength(s)+currentFileData.length,
+buffer.length);
+})
+.catch(throwNextTick);
+}
 
 // test that appendFile accepts buffers
 constfilename3=join(tmpdir.path,'append3.txt');
@@ -164,9 +177,8 @@ assert.throws(
 {code: 'ERR_INVALID_CALLBACK'});
 
 process.on('exit',function(){
-assert.strictEqual(10,ncallbacks);
+assert.strictEqual(ncallbacks,8);
 
-fs.unlinkSync(filename2);
 fs.unlinkSync(filename3);
 fs.unlinkSync(filename4);
 fs.unlinkSync(filename5);