[Tests] Prevent garbage generation from toolchain api test

[theotherjimmy]

Description

Prevent the garbage folders from showing up after running tests. They were caused by some compile command function generating directories.

Status

READY

Migrations

Nope!

Todos

• Tests
• Review by @bridadan
• Review by @geky

Steps to test or reproduce

Run py.test tools/test/toolchain/api.py

$ py.test tools/test/toolchains/api.py
========================================== test session starts ==========================================
platform linux2 -- Python 2.7.12, pytest-3.0.2, py-1.4.31, pluggy-0.3.1
rootdir: /home/jimbri01/src/mbedmicro/mbed, inifile: 
plugins: hypothesis-3.5.2, quickcheck-0.8.2
collected 5 items 

tools/test/toolchains/api.py .....

======================================= 5 passed in 2.24 seconds ========================================
$ ls
`                     [A?                   Jenkinsfile         #%\`RiS
~<                    aVqu                  K                   rtos
 ?                    CONTRIBUTING.md       k8 )STNnqN(bq0d     s1Ad
_=                    CP'@Ne                ?k?Tm}OJ            S9]j"MbYvY]hrnH1^?!0_rXy?^`=Z
!                     docs                  l                   SP\,
'                     DOXYGEN_FRONTPAGE.md  LICENSE             targets
(                     drivers               ;lJ^GDcS6qJV        ;*TBTz?KXoa9+
)]                    dRnW:?%               )l;?KWm?>D\ t*Jp"k  TESTS
[)                    E                     m$6S8l-t            tools
]                     E2ZJ#cI7`ME           mbed.h              u
{;                    }?e41                 mbed_settings.py    >}Ue
}                     events                mbed_settings.pyc   U%]}G$r
\                     #eW}o|n                n                  v
0                     features              N?                  (v
00                    fRw                   O                   %v
0\+1*18q0501          g                     \oa<:%?CNt          v{B:
0T                    G?I?5v                #@*oH{](2;>Y*       w
1                     +GoFLVp1d{U"yP        #o(j                w0
?\1Gfe88S?8A-:Zi0'r?  ^G>P\                 platform            x
2                     gp5o                  %pq                 XU#
2[sy                  h                     ;P?$uT"             xw
30                    H00                   Q                   XZww
5#5^m`S3p `E~$F&$z    hal                   ~qKN<               - \YIVJX?
6                     ia]?!-@&}D7EV~%1<J?e  RAkg                zDX
@?))7*>z              iqY#                  README.md
9                     j?                    re=qj[3_%*A
a                     jDzw%                 requirements.txt

[bridadan]

LGTM

[theotherjimmy]

@sg- LGTM!