Stubbing w/ more than 2 threads doesn't work

[linh-tran-mobosurvey]

If you stub a function with 3 mocks e.g.

stub someFunc \
"echo 1 >> somefile.txt" \
"echo 2 >> somefile.txt" \
"echo 3 >> somefile.txt";
sut_dosomething_thatwillcall_someFunc;
unstub someFunc

This will work if [sut_dosomething_thatwillcall_someFunc] is single threated. However,
if [sut_dosomething_thatwillcall_someFunc] has 3 threads, each thread calls [someFunc] once, the 3rd mock will never be executed (the 2nd mock "echo 2..." gets executed repetitively).

[jlmiller-crusoe]

@dynamotn's patch e0b2ca7 fixes an issue where if the test suite is killed 9 stub plans are still on-disk in the BATS_TMPDIR and are picked up after test suite modification, leading to unexpected behavior.

The fix in e0b2ca7 to change BATS_TMPDIR -> BATS_TEST_TMPDIR fixes my issue as well as #17 and since I was about to post a similar change to that directory please consider merging pull 56 @jasonkarns - #56 .