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+.

tests/test_block_header.c

@@ -320,7 +320,7 @@ test_lzma_block_header_encode(void)
 	uint8_t flags = out[1];
 
 	// Should have number of filters = 1
-	assert_uint_eq((flags & 0x3) + 1, 1);
+	assert_uint_eq((flags & 0x3) + 1U, 1);
 
 	// Bits 2-7 must be empty not set
 	assert_uint_eq(flags & (0xFF - 0x3), 0);