[NFC] Fix compiler warning in TestArmv7Disassembly.cpp

The warning is
    comparison of integers of different signs: 'const int' and 'const unsigned long'
and triggered by
    EXPECT_EQ (num_of_instructions, inst_list.GetSize());
as num_of_instructions is an int in this comparison (and the RHS is size_t).

llvm-svn: 341931