Testing: Add Unicode::GCString to the Docker installation

[bschmalhofer]

The test suite uses TAP::Harnesswhich often displays information about a failed test in tabular format. That formatting uses Unicode::GCStringwhen available. When the module is not available a message about that is shown.

not ok 12 - HumanReadableDataSize: Size => 58626123, Lang => ar_SA # TODO translation should be update to say: (MB)
# Failed test (with amnesty) 'HumanReadableDataSize: Size => 58626123, Lang => ar_SA'
# at /opt/otobo/scripts/test/Layout/HumanReadableDataSize.t line 148.
# Unicode::GCString is not installed, table may not display all unicode characters properly
# +------------------------------+----+------------------------------+
# | GOT                          | OP | CHECK                        |
# +------------------------------+----+------------------------------+
# | \N{U+200E}55.9 ��غابا�ت (MG) | eq | \N{U+200E}55.9 ��غابا�ت (MB) |
# +------------------------------+----+------------------------------+

I checked the documentation of Unicode::GCString and found that installing it should show no adverse effects. So, let's eliminate a confusing message and maybe get a better display of test violations.

[bschmalhofer]

The message "Unicode::GCString is not installed, table may not display all unicode characters properly" is gone now. Closing this issue.