Closes #113. This should be robust enough to handle the /bin => /usr/bin
symlink on CentOS7 as originally reported, but any manner of symlink
chains leading to the `bin/bats` executable in the installation
directory. Maintains portability between `readlink` implementations by
not relying on the GNU-specific `-e` option.

There's no perceptible performance impact.

This renames the original `bats_absolute_path` function to the more
obvious `bats_resolve_absolute_root_dir`, and introduces the
`BATS_MAX_SYMLINKS` variable to ensure symlink resolution short-circuits
in case cycles appear. While this can be user-controlled, I've not
documented it in the README, hoping that no one will need to change the
setting in a real-world situation.

Also tweaks the value of `BATS_TEST_SUITE_TMPDIR` from `tmp` to
`bats-test-tmp` to avoid potential collisions with other `BATS_TMPDIR`
directories.

I realized that the `while [[ -L "$PWD" && "$((--limit))" -ne -1 ]]`
loop could drop `limit` to `-1` before reaching the following check for
`[[ "$((--limit))" -ne -1 ]]`.

Added the "parent directories exhaust BATS_MAX_SYMLINKS" test case to
reproduce, which didn't cause an infinite loop, but failed when `cd`-ing
to `/usr/bin` terminated the inner loop, causing
`/usr/libexec/bats-core/bin/bats` not to be found.

Also added the "find BATS_ROOT without symlinks" test case to ensure the
loop executes correctly when `bin/bats` is executed directly, and added
`BATS_MAX_SYMLINKS=8` to "set BATS_ROOT with extreme symlink resolution"
to make sure the test traverses the exact number of expected symlinks.

Today I learned about the `set -P` option, which causes `cd` to set
`PWD` to the physical directory, containing no symlinks. This eliminated
the need for the inner `while` loop to resolve `PWD` symlinks.

The Bash man page wording is a little confusing, but there's a great
example demonstrating the option at:

  https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html

The result is the same, but subshells are only spawned to resolve
target_name links, making the code a little smaller and improving the
performance ever-so-slightly.

Should've tried this two commits ago. Verified manually that Bash parses
the options to `set` in order, such that you can safely disable and
renable an option in the same invocation.