Fix packing of values at offset 16.

Our bitmask for setting the highest bit to 0 for
0b11100000 (0xef) instead of 0b01111111 (0x7f) so
we would end up setting bit 5 of each offset to
zero. Fix this and expand test coverage by adding
a fake language (tgp) that has this bit set in both
its bytes.

This issue was discovered while adding CTS tests for
"tgl".

Change-Id: Ibb6de03000951c907c252049771039ab7466187a