Fix random test failure with make flags on test node

When running under testnode, there was sometimes this failure:

```
File "/srv/slapgrid/slappart8/srv/testnode/bee/inst/test0-0/parts/slapos.recipe.cmmi/slapos/recipe/cmmi/README.txt", line 521, in README.txt
Failed example:
    print(system(buildout))
Expected:
    Uninstalling package.
    Installing package.
    package: [ENV] CFLAGS = -I/sw/include
    package: [ENV] LDFLAGS = -L/sw/lib -L/some/extra/lib
    package: [ENV] TMP = /sample_buildout/parts/package/tmp
    package: Applying patches
    patching file configure
    patching file Makefile.dist
    patched-configure --prefix=/somewhere/else --with-threads --without-foobar
    building patched package
    installing patched package
    installing patched package-lib
    <BLANKLINE>
Got:
    Uninstalling package.
    Installing package.
    package: [ENV] CFLAGS = -I/sw/include
    package: [ENV] LDFLAGS = -L/sw/lib -L/some/extra/lib
    package: [ENV] TMP = /sample_buildout/parts/package/tmp
    package: Applying patches
    patching file configure
    patching file Makefile.dist
    patched-configure --prefix=/somewhere/else --with-threads --without-foobar
    building patched package
    installing patched package-lib
    installing patched package
    <BLANKLINE>
```

The order of in which `installing patched package-lib` and ` installing patched package` are printed is not always same.

This test does `make install install-lib` with a Makefile containing:

```
all:
        @echo building package

install:
        @echo installing package

install-lib:
        @echo installing package-lib
```

@vpelletier  and I investigated this and he pointed out that order of parts depends is not stable when make is invoked with `-j` option.

Turned out that [on test node](https://nexedi.erp5.net/test_result_module/20180308-55459479/7) `MAKEFLAGS` is  set to `-j12`.


/reviewed-on nexedi/slapos.recipe.cmmi!5

slapos/recipe/cmmi/tests.py

@@ -23,6 +23,7 @@ optionflags = (doctest.ELLIPSIS |
 
 
 def setUp(test):
+    os.environ.pop('MAKEFLAGS')
     zc.buildout.testing.buildoutSetUp(test)
     zc.buildout.testing.install('slapos.recipe.build', test)
     zc.buildout.testing.install_develop('slapos.recipe.cmmi', test)