Make run-test support multiple --with-agent flags

Previously we only supported a single --with-agent flag to be used
with run-test, silently ignoring additional uses. This makes run-test
support an arbitrary number of uses.

Test: ./test/run-test --host --dev 001-HelloWorld
Test: ./test/run-test --host \
                      --dev \
                      --with-agent libtifast.so=log,all \
                      001-HelloWorld
Test: ./test/run-test --host \
                      --dev \
                      --with-agent libtistress.so=jvmti-stress,redefine \
                      --with-agent libtifast.so=log,all \
                      001-HelloWorld

Change-Id: I3fe8538db64f79196037619a274842b3ab37a8d9