[fakes] fake-resultstore-wrap.sh

Factored out more reusable single-purpose scripts, for composing different
types of rsproxy tests.

fake-resultstore-wrap.sh is a command wrapper that starts and shuts down
a fake resultstore server, primarily used for integration testing.
It preserves the exit status of the wrapped command, verified by bazel
tests.

scripts/rs-ninja.sh is similar to rs-fakebuild.sh in that its main
purpose is to act as a command-prefix that responds to the RSPROXY_FIFO
environment variable set by rsproxy-wrap.sh -- they work together.
This design allows the proxy wrap and ninja invocation to be separated
by multiple layers of wrapping and functional composition.
rs-ninja.sh also generates a uuid for ninja to use as the invocation Id.
rs-ninja.sh is intended to be installed as part of the package.

Multiple scripts have been refactored to use rs-ninja.sh and
fake-resultstore-wrap.sh: test-rsproxy-wrap.sh, e2e_test.sh.

fakecas/server.go: resolves conflicting requirements in instance names
by adjusting when using the fake CAS from remote-apis-sdks.
This avoids an error from their fake library.

Generally, we are shaping scripts to be usable for both production
deployment and bazel-test usability.

Bug: 390427892
Change-Id: Id82cbdd4d809b7c97b14f95774a34e61e437fe71
Reviewed-on: https://fuchsia-review.googlesource.com/c/rsclient/+/1363477
Reviewed-by: Jay Zhuang <jayzhuang@google.com>
Commit-Queue: David Fang <fangism@google.com>