Migrate tool dependencies to go.tools.mod (#14646)

Go 1.14 introduces usage of alternate go.mod files to track
other dependencies which are not related to the main app.
We use this to track all tool dependencies so that everybody uses
the same version of all tools, including CI.

This will prevent version conflicts due to everybody using different
versions of the tools.
And it will not try to upgrade the tool version, every single time,
one runs the tool command.

While here, we also re-generate some filestore mocks which weren't up to date.

Fixes #13088

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

go.tools.mod

@@ -0,0 +1,10 @@
+module github.com/mattermost/mattermost-server/v5
+
+go 1.14
+
+require (
+	github.com/jstemmer/go-junit-report v0.9.1 // indirect
+	github.com/mattermost/mattermost-utilities v0.0.0-20200521071950-1be063e1b2fe // indirect
+	github.com/reflog/struct2interface v0.6.1 // indirect
+	github.com/vektra/mockery v1.1.2 // indirect
+)