Start work on fancy compiler tricks to expose extra stuff to our tests

This is mainly a matter of automake trickery: we build each static
library in two versions now: one with the TOR_UNIT_TESTS macro
defined, and one without.  When TOR_UNIT_TESTS is defined, we can
enable mocking and expose more functions. When it's not defined, we
can lock the binary down more.

The alternatives would be to have alternate build modes: a "testing
configuration" for building the libraries with test support, and a
"production configuration" for building them without.  I don't favor
that approach, since I think it would mean more people runnning
binaries build for testing, or more people not running unit tests.
Completely refactor how FILENAME_PRIVATE works

We previously used FILENAME_PRIVATE identifiers mostly for
identifiers exposed only to the unit tests... but also for
identifiers exposed to the benchmarker, and sometimes for
identifiers exposed to a similar module, and occasionally for no
really good reason at all.

Now, we use FILENAME_PRIVATE identifiers for identifiers shared by
Tor and the unit tests.  They should be defined static when we
aren't building the unit test, and globally visible otherwise. (The
STATIC macro will keep us honest here.)

For identifiers used only by the unit tests and never by Tor at all,
on the other hand, we wrap them in #ifdef TOR_UNIT_TESTS.

This is not the motivating use case for the split test/non-test
build system; it's just a test example to see how it works, and to
take a chance to clean up the code a little.
Coverage support: build with --enable-coverage to have tests run with gcov

If you pass the --enable-coverage flag on the command line, we build
our testing binaries with appropriate options eo enable coverage
testing.  We also build a "tor-cov" binary that has coverage enabled,
for integration tests.

On recent OSX versions, test coverage only works with clang, not gcc.
So we warn about that.

Also add a contrib/coverage script to actually run gcov with the
appropriate options to generate useful .gcov files.  (Thanks to
automake, the .o files will not have the names that gcov expects to
find.)

Also, remove generated gcda and gcno files on clean.
Add a script to compare directories full of gcov output

We can't just use "diff", since we don't care about changes in line
numbers, or changes in the exact number of times a line was called.
We just care about changes that make lines covered or non-coverd.  So
pre-process the files before calling diff.
Add rudimentary test mocking support.

This is not the most beautiful possible implementation (it requires
decorating mockable functions with ugly macros), but it actually
works, and is portable across multiple compilers and architectures.
Add some basic unit tests for the circuit map data structure.

These show off the new mocking code by mocking the circuitmux code
so that we can test the circuit map code in isolation.
Avoid compiler warning 'unused param'.

Fixes #9261.
Traditional /bin/sh is unhappy about {a,b} globbing.
Merge remote-tracking branch 'public/fancy_test_tricks'

Conflicts:
	src/common/include.am

Conflict was from adding testsupport.h near where sandbox.h had
already been added.
Merge remote-tracking branch 'linus/binshify'
Merge remote-tracking branch 'linus/unused_param'
Don't build format_helper_exit_status on win32

The only thing that used format_helper_exit_status on win32 was the
unit tests. This caused an error when we tried to leave a static
format_helper_exit_status lying around in a production object file.

The easiest solution is to admit that this way of dealing with process
exit status is Unix-only.