Add more assertions in `test_uplus_minus` #605

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	8m 1s
make (check, i686)	10m 17s
make (check, --disable-yjit)	7m 0s
make (check, --enable-shared --enable-load-relative)	7m 47s
make (test-bundler-parallel)	23m 41s
make (test-bundled-gems)	17m 50s
make (check, ubuntu-20.04)	8m 28s
Ubuntu result	1s
	1h 23m 5s

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