Add EML parsing from string to tests

A new test `TestEMLToMsgFromString` was added to "eml_test.go". This test asserts the proper functionality of `EMLToMsgFromString` method that allows us to parse EMLs directly from a string input. This test is a necessary part of ensuring the functionality and reliability of our EML parsing process.

eml_test.go

@@ -0,0 +1,65 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+package mail
+
+import (
+	"strings"
+	"testing"
+)
+
+const (
+	exampleMailPlainNoEnc = `Date: Thu, 14 Sep 2023 14:35:28 +0200
+MIME-Version: 1.0
+Message-ID:
+ <1305604950.683004066175.AAAAAAAAaaaaaaaaB@test.com>
+Subject: Example mail Plain text no Encoding
+User-Agent: go-mail v0.3.9 // https://github.com/wneessen/go-mail
+X-Mailer: go-mail v0.3.8 // https://github.com/wneessen/go-mail
+From: "Toni Tester" <go-mail@go-mail.dev>
+To: "Go Mail" <go-mail+test@go-mail.dev>
+Cc: Second Recipient <recipient@test.com>
+Bcc: "Invisible User" <bcc@bcc.de>
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+Dear Customer,
+
+This is a test mail. Please do not reply to this.
+
+
+Thank your for your business!
+The go-mail team
+
+-- 
+This is a signature`
+)
+
+func TestEMLToMsgFromString(t *testing.T) {
+	tests := []struct {
+		name string
+		eml  string
+		enc  string
+		sub  string
+	}{
+		{
+			"Plain text no encoding", exampleMailPlainNoEnc, "8bit",
+			"Example mail Plain text no Encoding",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			m, err := EMLToMsgFromString(tt.eml)
+			if err != nil {
+				t.Errorf("failed to parse EML: %s", err)
+			}
+			if m.Encoding() != tt.enc {
+				t.Errorf("EMLToMsgFromString failed: expected encoding: %s, but got: %s", tt.enc, m.Encoding())
+			}
+			if s := m.GetGenHeader(HeaderSubject); len(s) > 0 && !strings.EqualFold(s[0], tt.sub) {
+				t.Errorf("EMLToMsgFromString failed: expected subject: %s, but got: %s", tt.sub, s[0])
+			}
+		})
+	}
+}