Accidentally added a test that doesn't exist

Whoops!
give vcddiff the option to raise up module levels in VCD file

When diffing a reference VCD file against a VCD file generated by a
testbench, the signal names might be mismatched because the signals
in the testbench VCD might be one level down.

For instance, if the name in the reference VCD is "module::signal", the
name in the testbench-generated VCD might be
"module_tb::module::signal". This commit allows vcddiff to "hoist" the
signals in either of the VCD files up some number of levels.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>