Update Compute 'TestInit` test to use 'threadsafe.Buffer' (#1601)

### Change summary

This PR eliminates the intermittent behavior of the `TestInit` function
failing on the `windows-latest` test, which was caused by a race
condition associated with the CLI `yacspin spinner`. Replacing the
output capture from `bytes.Buffer` to `threadsafe.Buffer` ensures that
race conditions are no longer occur by removing potential unsafe access
to the buffer.

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* [x] Have you followed the guidelines in our Contributing document?
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Requests](https://github.com/fastly/cli/pulls) for the same
update/change?

Author: rcaril

pkg/commands/compute/init_test.go

@@ -1,7 +1,6 @@
 package compute_test
 
 import (
-	"bytes"
 	"context"
 	"errors"
 	"io"
@@ -454,7 +453,7 @@ func TestInit(t *testing.T) {
 				}
 			}
 
-			var stdout bytes.Buffer
+			var stdout threadsafe.Buffer
 			app.Init = func(_ []string, _ io.Reader) (*global.Data, error) {
 				opts := testutil.MockGlobalData(testcase.args, &stdout)
 				opts.Config = testcase.configFile