Sign gbl with test keys

Sign the built artifacts with test keys. These are purely for debugging
and development purposes.
Production (formal) builds should be resigned with prod keys.

Test keys generated with:
  openssl genrsa -traditional 4096 > testkey_RSA4096.pem
  openssl rsa -in testkey_RSA4096.pem -RSAPublicKey_out > testkey_RSA4096.pub.pem

Bug: 410495718
Bug: 387241522
Test: ./tools/bazel run //bootable/libbootloader:gbl_efi_dist
Test: for f in out/gbl_efi/gbl_*; ./bazel-bin/external/gbl/signtool/gblsigntool verify --key bootable/libbootloader/gbl/signtool/testkeys/testkey_RSA4096.pub.pem $f
Test: presubmit boot tests
Merged-In: Icc12e1573210f401de3bf95005c67f86888a24bc
Change-Id: Icc12e1573210f401de3bf95005c67f86888a24bc