units: add SHRINK parameter to testing.mak

If SHRINK=1 is specified as argument for units target of testing.mak,
and a test case if failed or timed out, shurink action is automaically
taken.

Here is an example (A bug is embedded for produce this exmpale):

    $ make -f testing.mak units SHRINK=1 UNITS_TIMEOUT=1
    Testing test.py as Python                                   passed
    Testing test.vhd as VHDL                                    failed (TIMED OUT)
    Shringking Units/test.vhd.t/input.vhd as VHDL
    [0     192381]...timeout(124)
    [0          0]...ok(0)
    [0     192381]...timeout(124)
    [0      96191]...timeout(124)
    [0      48096]...timeout(124)
    ...
    [158      160]...timeout(124)
    [159      160]...timeout(124)
    Minimal badinput input: Units/test.vhd.t/SHRINK-VHDL.tmp
    ............................................................
    a
    Testing traffic_signal.v as Verilog                         passed

Signed-off-by: Masatake YAMATO <yamato@redhat.com>