Fix "pkcs11" system test

Fix "pkcs11" system test

  - Define the SLOT environment variable before starting the test.  This
    variable defaults to 0 and that does not work with SoftHSM 2.

  - The system test expects the PIN environment variable to be set to
    "1234" while bin/tests/prepare-softhsm2.sh sets it to "0000".
    Update bin/tests/prepare-softhsm2.sh so that it sets the PIN to
    "1234".

  - Move contents of bin/tests/system/pkcs11/prereq.sh to
    bin/tests/system/pkcs11/setup.sh as the former was creating a file
    called "supported" that was getting removed by the latter before
    bin/tests/system/pkcs11/tests.sh could access it.

  - Fix typo in "have_ecx".