fix(libstore-tests): remove use-after-free bug for StringSource

Unfortunately `StringSource` class is very easy was very easy to misuse
because the ctor took a plain `std::string_view` which has a bad habit
of being implicitly convertible from an rvalue `std::string`. This lead
to unintentional use-after-free bugs.

This patch makes `StringSource` much harder to misuse by disabling the ctor
from a `std::string &&` (but `const std::string &` is ok).

Fix affected tests from libstore-tests.
Reformat those tests with clangd's range formatting since the diff is tiny
and it seems appropriate.

(cherry picked from commit 5bc8957c73)

src/libutil/serialise.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include <memory>
+#include <type_traits>
 
 #include "types.hh"
 #include "util.hh"
@@ -192,7 +193,14 @@ struct StringSource : Source
 {
     std::string_view s;
     size_t pos;
+
+    // NOTE: Prevent unintentional dangling views when an implicit conversion
+    // from std::string -> std::string_view occurs when the string is passed
+    // by rvalue.
+    StringSource(std::string &&) = delete;
     StringSource(std::string_view s) : s(s), pos(0) { }
+    StringSource(const std::string& str): StringSource(std::string_view(str)) {}
+
     size_t read(char * data, size_t len) override;
 };