cmdtest: end-to-end test for commands, flags and environment variables

There was no easy way to automatically test the end-to-end functionality
of commands, flags, environment variables etc.

The need for end-to-end testing was highlighted by the issues fixed
in #5341. There was no automated test to continually verify current
behaviour, nor a framework to quickly test the correctness of the fixes.

This change adds an end-to-end testing framework in the cmdtest folder.
It has some simple examples in func TestCmdTest in cmdtest_test.go. The
tests should be readable by anybody familiar with rclone and look like
this:

    // Test the rclone version command with debug logging (-vv)
    out, err = rclone("version", "-vv")
    if assert.NoError(t, err) {
        assert.Contains(t, out, "rclone v")
        assert.Contains(t, out, "os/version:")
        assert.Contains(t, out, " DEBUG : ")
    }

The end-to-end tests are executed just like the Go unit tests, that is:

    go test ./cmdtest -v

The change also contains a thorough test of environment variables in
environment_test.go.

Thanks to @ncw for encouragement and introduction to the TestMain trick.

cmdtest/cmdtest.go

@@ -0,0 +1,19 @@
+// Package cmdtest creates a testable interface to rclone main
+//
+// The interface is used to perform end-to-end test of
+// commands, flags, environment variables etc.
+//
+package cmdtest
+
+// The rest of this file is a 1:1 copy from rclone.go
+
+import (
+	_ "github.com/rclone/rclone/backend/all" // import all backends
+	"github.com/rclone/rclone/cmd"
+	_ "github.com/rclone/rclone/cmd/all"    // import all commands
+	_ "github.com/rclone/rclone/lib/plugin" // import plugins
+)
+
+func main() {
+	cmd.Main()
+}