MM-16997: Simplified TestUploadFiles (#11707)

* MM-16997: Simplified TestUploadFiles

Tickets:
https://mattermost.atlassian.net/browse/MM-16997
https://mattermost.atlassian.net/browse/MM-16760

- The tests now fully buffer the data vefore uploading it. The prior
  code was much more complex because it was intended to eventually
  mature into generic client code; not a priority, definitely not
  valuable for this test.
- Also improved error handling in TestHookFileWillBeUploaded for
  MM-16760

* PR feedback: typo

app/plugin_hooks_test.go

@@ -497,6 +497,7 @@ func TestHookFileWillBeUploaded(t *testing.T) {
 			package main
 
 			import (
+				"fmt"
 				"io"
 				"github.com/mattermost/mattermost-server/plugin"
 				"github.com/mattermost/mattermost-server/model"
@@ -507,7 +508,10 @@ func TestHookFileWillBeUploaded(t *testing.T) {
 			}
 
 			func (p *MyPlugin) FileWillBeUploaded(c *plugin.Context, info *model.FileInfo, file io.Reader, output io.Writer) (*model.FileInfo, string) {
-				output.Write([]byte("ignored"))
+				n, err := output.Write([]byte("ignored"))
+				if err != nil {
+					return info, fmt.Sprintf("FAILED to write output file n: %v, err: %v", n, err)
+				}
 				info.Name = "ignored"
 				return info, "rejected"
 			}
@@ -607,6 +611,7 @@ func TestHookFileWillBeUploaded(t *testing.T) {
 			import (
 				"io"
 				"bytes"
+				"fmt"
 				"github.com/mattermost/mattermost-server/plugin"
 				"github.com/mattermost/mattermost-server/model"
 			)
@@ -616,13 +621,17 @@ func TestHookFileWillBeUploaded(t *testing.T) {
 			}
 
 			func (p *MyPlugin) FileWillBeUploaded(c *plugin.Context, info *model.FileInfo, file io.Reader, output io.Writer) (*model.FileInfo, string) {
-				p.API.LogDebug(info.Name)
 				var buf bytes.Buffer
-				buf.ReadFrom(file)
-				p.API.LogDebug(buf.String())
+				n, err := buf.ReadFrom(file)
+				if err != nil {
+					return info, fmt.Sprintf("FAILED to read input file n: %v, err: %v", n, err)
+				}
 
 				outbuf := bytes.NewBufferString("changedtext")
-				io.Copy(output, outbuf)
+				n, err = io.Copy(output, outbuf)
+				if int(n) != len("changedtext") || err != nil {
+					return info, fmt.Sprintf("FAILED to write output file n: %v, err: %v", n, err)
+				}
 				info.Name = "modifiedinfo"
 				return info, ""
 			}