core: move basic chain creation into a package of its own

This allows to reuse it across different packages.

testchain can't be used because of circular dependencies.

Init() is not changed except for filepath.Join() use instead of direct string
appends which is a better approach anyway. rootpath is required because
current directory will change from package to package.

internal/basicchain/testdata/verify_args/verification_with_args_contract.go

@@ -0,0 +1,7 @@
+package verify_args
+
+// Verify is a verification contract method which takes several arguments.
+func Verify(argString string, argInt int, argBool bool) bool {
+	isOK := argString == "good_string" || argInt == 5 || argBool == true
+	return isOK
+}