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/libutil-tests/processes.cc

@@ -0,0 +1,17 @@
+#include "processes.hh"
+
+#include <gtest/gtest.h>
+
+namespace nix {
+
+/* ----------------------------------------------------------------------------
+ * statusOk
+ * --------------------------------------------------------------------------*/
+
+TEST(statusOk, zeroIsOk)
+{
+    ASSERT_EQ(statusOk(0), true);
+    ASSERT_EQ(statusOk(1), false);
+}
+
+} // namespace nix