msgb/test: Add test case for error cases

Include a test for msgb_trim.

Sponsored-by: On-Waves ehf
diff --git a/tests/msgb/msgb_test.ok b/tests/msgb/msgb_test.ok
index 4cb76a9..6603fe7 100644
--- a/tests/msgb/msgb_test.ok
+++ b/tests/msgb/msgb_test.ok
@@ -18,6 +18,8 @@
 Buffer: (L1=data-124) 00 00 00 00 00 00 00 00 [L2]> 00 00 00 00 [L3]> (L4=tail+4) 
 Buffer: (L1=data-124) 00 00 00 00 00 00 00 00 [L2]> (L3+8) 00 00 00 00 (L4=tail+4) 
 Buffer: (L1=data-124) 00 00 00 00 00 00 00 00 [L2]> 00 00 00 00 (L3 out of range) (L4=tail+4) 
+Testing the msgb API error handling
+msgb(%p): Negative length is not allowed
 Testing msgb_copy
 Src: [L1]> 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 [L2]> 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 [L3]> 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37 38 39 3a 3b [L4]> 3c 3d 3e 3f 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f 
 Dst: [L1]> 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 [L2]> 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 [L3]> 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37 38 39 3a 3b [L4]> 3c 3d 3e 3f 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f