Make most mojo_unittests pass with MojoIpcz enabled

Tweaks or skips tests within mojo_unittests so that they pass with
MojoIpcz enabled.

Many tests pass unmodified, but some require tweaks to test
expectations which were built around Mojo implementation details
rather than API specs.

Other tests simply don't make sense for MojoIpcz -- either due to
unused or deprecated Mojo features that MojoIpcz does not implement,
or due to tests which were designed to cover implementation details
not relevant to MojoIpcz -- and these tests are automatically passed
when the feature flag is enabled.

This only makes minor implementation tweaks, supporting unserialized
Mojo message APIs (which still eagerly serialize on MojoWriteMessage
regardless of peer remoteness), and very basic quota API as used by
some tests to inspect message queue state.

LOW_COVERAGE_REASON=Much of the coverage here comes in the form of existing unit tests which only provide coverage with a pending feature flag enabled. Coverage will improve substantially once the flag is available and enabled in test configs.

Bug: 1299283
Change-Id: I01ac6882a60b2ace9a0a319d2d4d705125524bdc
Binary-Size: This is linking in a new library of code (ipcz) previously unused in production. The size increase is unavoidable but temporary, as this new library will eventually replace a library of similar code size which can be dropped from the build.
Fuchsia-Binary-Size: See above note.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3848975
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1044291}