Add fresh concatStringsSep without bug

The buggy version was previously renamed to
dropEmptyInitThenConcatStringsSep

tests/unit/libutil/strings.cc

@@ -0,0 +1,83 @@
+#include <gtest/gtest.h>
+
+#include "strings.hh"
+
+namespace nix {
+
+using Strings = std::vector<std::string>;
+
+/* ----------------------------------------------------------------------------
+ * concatStringsSep
+ * --------------------------------------------------------------------------*/
+
+TEST(concatStringsSep, empty)
+{
+    Strings strings;
+
+    ASSERT_EQ(concatStringsSep(",", strings), "");
+}
+
+TEST(concatStringsSep, justOne)
+{
+    Strings strings;
+    strings.push_back("this");
+
+    ASSERT_EQ(concatStringsSep(",", strings), "this");
+}
+
+TEST(concatStringsSep, emptyString)
+{
+    Strings strings;
+    strings.push_back("");
+
+    ASSERT_EQ(concatStringsSep(",", strings), "");
+}
+
+TEST(concatStringsSep, emptyStrings)
+{
+    Strings strings;
+    strings.push_back("");
+    strings.push_back("");
+
+    ASSERT_EQ(concatStringsSep(",", strings), ",");
+}
+
+TEST(concatStringsSep, threeEmptyStrings)
+{
+    Strings strings;
+    strings.push_back("");
+    strings.push_back("");
+    strings.push_back("");
+
+    ASSERT_EQ(concatStringsSep(",", strings), ",,");
+}
+
+TEST(concatStringsSep, buildCommaSeparatedString)
+{
+    Strings strings;
+    strings.push_back("this");
+    strings.push_back("is");
+    strings.push_back("great");
+
+    ASSERT_EQ(concatStringsSep(",", strings), "this,is,great");
+}
+
+TEST(concatStringsSep, buildStringWithEmptySeparator)
+{
+    Strings strings;
+    strings.push_back("this");
+    strings.push_back("is");
+    strings.push_back("great");
+
+    ASSERT_EQ(concatStringsSep("", strings), "thisisgreat");
+}
+
+TEST(concatStringsSep, buildSingleString)
+{
+    Strings strings;
+    strings.push_back("this");
+
+    ASSERT_EQ(concatStringsSep(",", strings), "this");
+}
+
+} // namespace nix