Add a comment in bios-tables-test.c to clarify the reason behind approach

A comment is added in bios-tables-test.c that explains the reasoning
behind the process of updating the ACPI table blobs when new tests are added
or old tests are modified or code is committed that affect tests. The
explanation would help future contributors follow the correct process when
making code changes that affect ACPI tables.

Signed-off-by: Ani Sinha <ani@anisinha.ca>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20200929142501.1057-1-ani@anisinha.ca>
Signed-off-by: Thomas Huth <thuth@redhat.com>