Tests: Silence -Wsign-conversion warning on GCC version < 10.

Tests: Silence -Wsign-conversion warning on GCC version < 10.

Since GCC version 10, GCC no longer complains about simple implicit
integer conversions with Arithmetic operators.

For instance:

    uint8_t a = 5;
    uint32_t b = a + 5;

Give a warning on GCC 9 and earlier but this:

    uint8_t a = 5;
    uint32_t b = (a + 5) * 2;

Gives a warning with GCC 10+.