Move unit tests to the location Meson expects them to be

Everything that is a separate subproject should live in the subprojects
directory.

Progress on #2503

This reverts commit 451f8a8c19.

src/libstore-tests/s3-binary-cache-store.cc

@@ -0,0 +1,18 @@
+#if ENABLE_S3
+
+#  include <gtest/gtest.h>
+
+#  include "s3-binary-cache-store.hh"
+
+namespace nix {
+
+TEST(S3BinaryCacheStore, constructConfig)
+{
+    S3BinaryCacheStoreConfig config{"s3", "foobar", {}};
+
+    EXPECT_EQ(config.bucketName, "foobar");
+}
+
+} // namespace nix
+
+#endif