test: Fix Wambiguous-reversed-operator compiler warnings #24406

Add a missing const to avoid the C++20 clang compiler warning:

test/fuzz/addrman.cpp:325:22: error: ISO C++20 considers use of overloaded operator '==' (with operand types 'AddrManDeterministic' and 'AddrManDeterministic') to be ambiguous despite there being a unique best viable function [-Werror,-Wambiguous-reversed-operator]
    assert(addr_man1 == addr_man2);
           ~~~~~~~~~ ^  ~~~~~~~~~
/usr/include/assert.h:93:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
test/fuzz/addrman.cpp:140:10: note: ambiguity is between a regular call to this operator and a call with the argument order reversed
    bool operator==(const AddrManDeterministic& other)
         ^
1 error generated.

This patch also fixes the compile error if the first operand is const:

test/fuzz/addrman.cpp:326:23: error: invalid operands to binary expression ('const AddrManDeterministic' and 'AddrManDeterministic')
    assert(addr_man_1 == addr_man2);
           ~~~~~~~~~~ ^  ~~~~~~~~~
/usr/include/assert.h:90:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
test/fuzz/addrman.cpp:140:10: note: candidate function not viable: 'this' argument has type 'const AddrManDeterministic', but method is not marked const
    bool operator==(const AddrManDeterministic& other)
         ^
1 error generated.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #24169 (build: Add --enable-c++20 option by MarcoFalke)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

ACK fafc4eb3637be0a85644c89c355fe68678a62c17, I have reviewed the code and it looks OK, I agree it can be merged.

New comparison operator signatures follow reference prototypes.

Did not test with actual compiling with std=c++20.