tests/bios-tables-test: fix assert

tests/bios-tables-test: fix assert

Newer iasl does not add the aml file name to the Definition Block.
See acpica tools commit  1ecbb3d5:
  "Emit the AMLFilename as a zero-length string. Allows the compiler to create
   the name later -- making it easier to rename the parent ASL (DSL) file."

That causes an assert in acpi tests:
   tests/bios-tables-test.c:455:normalize_asl: assertion failed: (block_name)

Fix it by striping the start of the definition block line until the first comma.
The block name is always the first parameter and
the grammar does not allow comma in between, so it is safe.

Reported-by: Dr. David Alan Gilbert <[email protected]>
Signed-off-by: Marcel Apfelbaum <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>