WIP: Allow sourcing of helper files from BATS_LIB_PATH

[ntnn]

The existing functionality of load() is preserved:
1. Load helper files in the same directory the current testfile
resides in
2. Load helper files by absolute path

Additionally an environment variable BATS_LIB_PATH can be defined to
be source helper files from.

If BATS_LIB_PATH is empty the following locations are used:
1. $HOME/.bats/lib
Allows users to manage helper libraries themselves, similar to
python/ruby/...
2. /usr/lib/bats
Allows to install helper libraries via a package manager.

Example usage:

• bats installed via package manager
• bats-support files installed via package manager to /usr/lib/bats

$ tree /usr/lib/bats
/usr/lib/bats
├── bats-support/
│   ├── error.bash
│   ├── lang.bash
│   └── output.bash
└── bats-support.bash

1 directory, 4 files
$ cat /usr/lib/bats/bats-support
source "$(dirname "${BASH_SOURCE[0]}")/bats-support/output.bash"
source "$(dirname "${BASH_SOURCE[0]}")/bats-support/error.bash"
source "$(dirname "${BASH_SOURCE[0]}")/bats-support/lang.bash"
$ cat example.bats
#!/usr/bin/env bats

load bats-support
@test "This will fail" {
    fail "This is an error message with error.bash loaded from /usr/lib/bats/error.bash"
}
$ bats example.bats
1..1
not ok 1 This will fail
# (from function `fail' in file /usr/lib/bats/bats-support/error.bash, line 40,
#  in test file example.bats, line 6)
#   `fail "This is an error message with error.bash loaded from /usr/lib/bats/error.bash"' failed
# This is an error message with error.bash loaded from /usr/lib/bats/error.bash

The bats-support here is different than the load.bash found in the repository.

Currently a loading file is required, which sources the files in the directory, unless the library is a single file.
To work around this I'll add a mechanism which sources all files in a directory, if the directory exists but no loading file is found.

Also, the last line of the error message shows the wrong location for error.bash.

• Fix error message with wrong path
• Source directory if no loading file exists

[ntnn]

@ztombol You've written three libraries for bats so far, what do you think of a load mechanism like this?

Additionally I've read quite some issues and comments about loading helper files, but none seem to have fruited into anything other than the current load(), which only sources in the same dir or absolute paths.

[ntnn]

Additionally I'd like to know how heavy posix compliance weighs in. Since many features of bats are based on non-posix functionality (e.g. set +-E/set +-T) it doesn't make sense to use the slower [ builtin vs. the [[ keyword.

If POSIX-compliance is a long term goal I'll gladly push that to the branch.