Add run script to execute eunit tests

Usage is the same as for test/etap/run:

    ./test/couchdb/run -v ${PATH}

-v runs in verbose mode, as etap does. Also, you can use make for that:

    make check-eunit

which will run tests everywhere in project where Makefile contains
check-eunit subcommand definition.

The ${PATH} thing could be single file or directory. The latter should
contains *_tests.erl files which would be compiled and executed by
eunit.

The *_tests.erl

The reason of compiling on run instead of using autoconf for that is
to simplify tests developing and avoid situations, when you'd fixed
the test or add new one, but forgot to remove/compile beam file.

All test_*.beam files are been stored in test/couchdb/ebin directory,
the temporary test files will be places to test/couchdb/temp one.
Both directories will be removed by make clean/distclean command.
Add common header for eunit test files
Port 001-load.t etap test suite to eunit
Port 002-icu-driver.t etap test suite to eunit

See setup/0 comment for specific info about loading
couch_icu_driver with eunit.
Port 010-file-basics.t and 011-file-headers.t etap test suites to eunit

Both merged into single suite since they tests single target and shares
common bits.
Port 020-btree-basics.t and 021-btree-reductions.t etap suites to eunit

Both merged into single suite since they tests single target and shares
common bits.
Port 030-doc-from-json.t and 031-doc-to-json.t etap suites to eunit

Both merged into single suite since they tests single target and shares
common bits.
Port 040-util.t etap test suite to eunit
Port 041-uuid.t etap test suite to eunit

Config files are removed in favor of using couch_config API instead.
Port 042-work-queue.t etap test suite to eunit

Etap tests were made in flow style, testing the same things multiple
times without real need. For eunit they are split into small test cases
to focus on testing goals.

Timeout on receive is decreased from 3000 to 100.
Port 043-find-in-binary.t etap test suite to eunit

It been merged into couch_util_tests suite.
Port 050-stream.t etap test suite to eunit
Port 06[0-5]-kt-*.t etap test suites to eunit

All merged into single suite since they're test the same module.
Port 070-couch-db.t etap test suite to eunit

Fix ERL_LIB environment variable setting. Add ?tempdb macros for
unique temporary database name generation.
Port 072-cleanup.t etap test suite to eunit

requests functions from test_util were moved to test_request module
with nicer and simpler API.
Port 073-changes.t etap test suite to eunit

For heartbeats test they don't being counted anymore since their
amount is heavy depends from the overall system performance and some
assertions may fail or not because of that. Instead of this, we just
ensure that their amount is going to increase over the time.
Port 074-doc-update-conflicts.t etap test suite to eunit

Timeout decreased, added 10K clients case
Port 075-auth-cache.t etap test suite to eunit

Timeouts are removed.
Port 076-file-compression.t etap test suite to eunit

The original test suite was decoupled into compaction and comparison
cases.
Port 077-couch-db-fast-db-delete-create.t etap test suite to eunit

Merged into couch_db_tests suite.