cmd/geth: make test pass on a pi4 by using lightkdf

[ligi]

before this change this test was failing reproducibly with:

--- FAIL: TestAccountImport (0.00s)
    --- FAIL: TestAccountImport/correct_account (5.08s)
        test_cmd.go:262: (stderr:4) INFO [01-30|04:50:50.619] Maximum peer count                       ETH=50 LES=0 total=50
        test_cmd.go:262: (stderr:4) INFO [01-30|04:50:50.619] Smartcard socket not found, disabling    err="stat /run/pcscd/pcscd.comm: no such file or directory"
        test_cmd.go:262: (stderr:4) WARN [01-30|04:50:50.622] Lowering memory allowance on 32bit arch  available=3838 addressable=2048
        test_cmd.go:262: (stderr:4) WARN [01-30|04:50:50.622] Sanitizing cache to Go's GC limits       provided=1024 updated=682
        test_cmd.go:262: (stderr:4) INFO [01-30|04:50:50.623] Set global gas cap                       cap=50,000,000
        test_cmd.go:242: killing the child process (timeout)
        test_cmd.go:116: not enough output, got until ◊:
            ---------------- (stdout text)
            
            ---------------- (expected text)
            ◊Address: {fcad0b19bb29d4674531d6f115237e16afce377c}

after this change this tests passes on a pi4 and it should also not harm the test to do lightkdf here

[holiman]

Lgtm, I changed to use lighkdf in other places like this, to lower the mem usage during testruns.