refactor(pkg): refactor Plugin to make it easier to test

Writing the plugin's output string and exiting the program will no
longer take place in the plugin status itself. It will only be done in
the main program.

pkg/plugin/plugin.go

@@ -5,7 +5,6 @@ package plugin // import nocternity.net/gomonop/pkg/perfdata
 import (
 	"container/list"
 	"fmt"
-	"os"
 	"strings"
 
 	"nocternity.net/gomonop/pkg/perfdata"
@@ -70,10 +69,9 @@ func (p *Plugin) AddPerfData(pd *perfdata.PerfData) {
 	p.perfData[pd.Label] = pd
 }
 
-// Done generates the plugin's text output from its name, status, text data
-// and performance data, before exiting with the code corresponding to the
-// status.
-func (p *Plugin) Done() {
+// String generates the plugin's text output from its name, status, text data
+// and performance data.
+func (p *Plugin) String() string {
 	var strBuilder strings.Builder
 	strBuilder.WriteString(p.name)
 	strBuilder.WriteString(" ")
@@ -99,6 +97,10 @@ func (p *Plugin) Done() {
 			strBuilder.WriteString(em.Value.(string))
 		}
 	}
-	fmt.Println(strBuilder.String())
-	os.Exit(int(p.status))
+	return strBuilder.String()
+}
+
+// ExitCode returns the plugin's exit code.
+func (p *Plugin) ExitCode() int {
+	return int(p.status)
 }

pkg/program/program.go

@@ -1,9 +1,11 @@
 package program // import nocternity.net/gomonop/pkg/program
 
+import "nocternity.net/gomonop/pkg/plugin"
+
 type Program interface {
+	Output() *plugin.Plugin
 	CheckArguments() bool
 	RunCheck()
-	Done()
 }
 
 type Builder func() Program