Add unit tests for new invariants:

- NoModifiedUnmodifiableFields and ValidPseudoAccounts
- Move the Invariants_test class into the test namespace

include/xrpl/protocol/STLedgerEntry.h

@@ -26,7 +26,9 @@
 namespace ripple {
 
 class Rules;
+namespace test {
 class Invariants_test;
+}
 
 class STLedgerEntry final : public STObject, public CountedObject<STLedgerEntry>
 {
@@ -86,7 +88,8 @@ private:
     void
     setSLEType();
 
-    friend Invariants_test;  // this test wants access to the private type_
+    friend test::Invariants_test;  // this test wants access to the private
+                                   // type_
 
     STBase*
     copy(std::size_t n, void* buf) const override;