MPTTester::operator() parameter should be std::int64_t

- Originally defined as uint64_t, but the testIssuerLoan() test called
  it with a negative number, causing an overflow to a very large number
  that in some circumstances could be silently cast back to an int64_t,
  but might not be. I believe this is UB, and we don't want to rely on
  that.

src/test/jtx/impl/mpt.cpp

@@ -644,7 +644,7 @@ MPTTester::operator[](std::string const& name) const
 }
 
 PrettyAmount
-MPTTester::operator()(std::uint64_t amount) const
+MPTTester::operator()(std::int64_t amount) const
 {
     return MPT("", issuanceID())(amount);
 }

src/test/jtx/mpt.h

@@ -272,7 +272,7 @@ public:
     operator[](std::string const& name) const;
 
     PrettyAmount
-    operator()(std::uint64_t amount) const;
+    operator()(std::int64_t amount) const;
 
     operator Asset() const;