connect: fix strategy testing for attempts, timeouts and happy-eyeball

- add test2600 as a unit test that triggers various connect conditions
  and monitors behaviour, available in a debug build only.

- this exposed edge cases in connect.c that have been fixed

Closes #10312

tests/data/Makefile.inc

@@ -248,6 +248,8 @@ test2400 test2401 test2402 \
 \
 test2500 test2501 test2502 \
 \
+test2600 \
+\
 test3000 test3001 test3002 test3003 test3004 test3005 test3006 test3007 \
 test3008 test3009 test3010 test3011 test3012 test3013 test3014 test3015 \
 test3016 test3017 test3018 test3019 test3020 test3021 test3022 test3023 \

tests/data/test2600

@@ -0,0 +1,24 @@
+<testcase>
+<info>
+<keywords>
+unittest
+llist
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+<features>
+unittest
+debug
+http
+</features>
+ <name>
+llist unit tests
+ </name>
+</client>
+</testcase>