Closes #34. Incorporates the test code from that PR.

This fixes a problem whereby invoking an exported function within a test
script would result in an incorrect stack trace. It does this by using
`BATS_TEST_SOURCE` to capture a function's frame when its corresponding
`BASH_SOURCE` entry is empty.

Note: I believe the earlier problem described in sstephenson/bats#225
whereby no output was generated at all was fixed by commit
0f6dde5.

My investigation revealed that in Bash versions all the way up through
4.3.48, before this code change, the test failed as @pawamoy reported,
with the underlying test fixture producing the following output:

   not ok 1 failing test
     (in file , line 7,
      from function `bats_perform_test' in file libexec/bats-exec-test, line 339,
      from function `main' in test file , line 391)

Starting with Bash 4.4, the test passed.

The apparent reason for the failure is that the `BASH_SOURCE` entry
corresponding to the test file was getting wiped out when the exported
function was called. (Once I converged on this as a potential culprit, I
verified it by printing the contents of `BASH_SOURCE` before and after
the exported function call.) Since `bats_capture_stack_trace` depends on
matching a `BASH_SOURCE` value against `BATS_TEST_SOURCE`, when the
`BASH_SOURCE` corresponding to the test function was empty, it would
keep collecting stack frames, resulting in the output above.

As best I can currently tell,  the change accounting for the fix in Bash
4.4 is (from the Bash 4.4 ChangeLog):

  1/2/2014
  --------
  ...snip...

  2/26
  ----
  [bash-4.3 released]
  ...snip...

  11/28
  -----
  execute_cmd.c
    - struct func_array_state: new state to save state of BASH_LINENO,
      BASH_SOURCE, and FUNCNAME during function execution so it can be
      restored on a jump to top level
    - restore_funcarray_state: new function to restore func_array_state
    - execute_function: fill in func_array_state variable, add unwind-
      protect to restore it on jump to top level, call explicitly at
      end of function if subshell != 0 (may not be necessary, but safe
      for now).  Fixes bug with local assignments to FUNCNAME reported
      by Arfrever Frehtes Taifersar Arahesis <arfrever.fta@gmail.com>

  1/1/2015
  --------
  ...snip...

  7/7
  ---
  ...snip...
  [bash-4.4-alpha frozen]

This makes it faster to iterate on running changes to Bats inside the
container, since the apk packages are far less likely to change.

Should've done this in the previous commit rather than using a regex,
but I'd forgotten until I looked over other test cases after-the-fact.

This allows the "execute exported function without breaking failing test
output" to pass both locally/natively and in the Alpine Docker
container.