[mdatagen] bugfix: fix generated package test when skipping goleak, b…

…ecause os package import is forgotten (#10486)

#### Description

When `tests.goleak.skip` flag is set to true, the
generated_package_test.go is missing the `import os` statement. I
added the missing import in this PR.

#### Testing

Compare the generated generated_package_test.go when tests.goleak.skip
flag is toggle from false to true.

**before**

```diff
--- a/receiver/runnreceiver/generated_package_test.go
+++ b/receiver/runnreceiver/generated_package_test.go
@@ -3,10 +3,10 @@
 package runnreceiver

 import (
-       "go.uber.org/goleak"
        "testing"
 )

 func TestMain(m *testing.M) {
-       goleak.VerifyTestMain(m)
+       // skipping goleak test as per metadata.yml configuration
+       os.Exit(m.Run())
 }
```

**after**

```diff
--- a/receiver/runnreceiver/generated_package_test.go
+++ b/receiver/runnreceiver/generated_package_test.go
@@ -3,10 +3,11 @@
 package runnreceiver

 import (
-       "go.uber.org/goleak"
+       "os"
        "testing"
 )

 func TestMain(m *testing.M) {
-       goleak.VerifyTestMain(m)
+       // skipping goleak test as per metadata.yml configuration
+       os.Exit(m.Run())
 }
```

cmd/mdatagen/templates/package_test.go.tmpl

@@ -3,6 +3,9 @@
 package {{ if isCommand -}}main{{ else }}{{ .Package }}{{- end }}
 
 import (
+    {{- if .Tests.GoLeak.Skip }}
+    "os"
+    {{- end }}
 	"testing"
 
     {{- if not .Tests.GoLeak.Skip }}