Cargo: build the tests against rust 2015

The library is building against rust 2018 but the test can't
compile succesfully against that.

Cargo.toml

@@ -40,3 +40,55 @@ libc = "0.2"
 
 [workspace]
 members=["mixed_tests" ]
+
+[[test]]
+name = "argument-types"
+edition = "2015"
+
+[[test]]
+name = "basic"
+edition = "2015"
+
+[[test]]
+name = "c_derive"
+edition = "2015"
+
+[[test]]
+name = "drop"
+edition = "2015"
+
+[[test]]
+name = "floating-refs"
+edition = "2015"
+
+[[test]]
+name = "interfaces"
+edition = "2015"
+
+[[test]]
+name = "memory-layout"
+edition = "2015"
+
+[[test]]
+name = "no-instance-private"
+edition = "2015"
+
+[[test]]
+name = "override"
+edition = "2015"
+
+[[test]]
+name = "properties"
+edition = "2015"
+
+[[test]]
+name = "signals"
+edition = "2015"
+
+[[test]]
+name = "gir"
+edition = "2015"
+
+[[test]]
+name = "variant"
+edition = "2015"