tests: threads: Test if grandchild message promotes only its subthread

lib/tests/testdir3/sort/grandchild-promotes-only-subthread/cur/A

@@ -0,0 +1,7 @@
+From: testfrom@example.com
+To: testto@example.com
+Subject: A
+Message-Id: <A@msg.id>
+Date: Sat, 17 May 2014 10:00:00 +0000
+
+A

lib/tests/testdir3/sort/grandchild-promotes-only-subthread/cur/B

@@ -0,0 +1,7 @@
+From: testfrom@example.com
+To: testto@example.com
+Subject: B
+Message-Id: <B@msg.id>
+Date: Sat, 17 May 2014 10:00:00 +0000
+
+B

lib/tests/testdir3/sort/grandchild-promotes-only-subthread/cur/C

@@ -0,0 +1,9 @@
+From: testfrom@example.com
+To: testto@example.com
+Subject: C
+Message-Id: <C@msg.id>
+References: <B@msg.id>
+In-reply-to: <B@msg.id>
+Date: Sat, 17 May 2014 10:00:00 +0000
+
+C

lib/tests/testdir3/sort/grandchild-promotes-only-subthread/cur/D

@@ -0,0 +1,9 @@
+From: testfrom@example.com
+To: testto@example.com
+Subject: D
+Message-Id: <D@msg.id>
+References: <B@msg.id>
+In-reply-to: <B@msg.id>
+Date: Sat, 17 May 2014 10:00:00 +0000
+
+D

lib/tests/testdir3/sort/grandchild-promotes-only-subthread/cur/E

@@ -0,0 +1,9 @@
+From: testfrom@example.com
+To: testto@example.com
+Subject: E
+Message-Id: <E@msg.id>
+References: <B@msg.id>
+In-reply-to: <B@msg.id>
+Date: Sat, 17 May 2014 10:00:00 +0000
+
+E

lib/tests/testdir3/sort/grandchild-promotes-only-subthread/cur/F

@@ -0,0 +1,9 @@
+From: testfrom@example.com
+To: testto@example.com
+Subject: F
+Message-Id: <F@msg.id>
+References: <B@msg.id> <D@msg.id>
+In-reply-to: <D@msg.id>
+Date: Sat, 17 May 2014 10:00:00 +0000
+
+F

lib/tests/testdir3/sort/grandchild-promotes-only-subthread/cur/G

@@ -0,0 +1,7 @@
+From: testfrom@example.com
+To: testto@example.com
+Subject: G
+Message-Id: <G@msg.id>
+Date: Sat, 17 May 2014 10:00:00 +0000
+
+G