test: del: test warning suppressed on removing conflict constraint

[TT: cherry-picked test, rewrote commit message]

test/solver/installif13.installed

@@ -0,0 +1,18 @@
+C:Q1EyN5AdpAOBJWKMR89pp/C66o+OE=
+P:app
+V:1
+S:1
+I:1
+D:lib
+
+C:Q1eVpkasfqZAukAXFYbgwt4xAMZWU=
+P:lib
+V:1
+S:1
+I:1
+
+C:Q1C4uoV7SdMdDhYg4OCVmI71D8HIA=
+P:foo
+V:1
+S:1
+I:1

test/solver/installif13.test

@@ -0,0 +1,7 @@
+@ARGS del appiif1
+@REPO installif1.repo
+@INSTALLED installif13.installed
+@WORLD app foo !appiif1
+@EXPECT
+(1/1) Installing appiif1 (1)
+OK: 0 MiB in 4 packages