add unit test to catch bug in EB v1.11.0 related to $LMOD_CMD and spider

this is a missing unit test for the bug fixed in #861

this test was added to catch issue with path to `lmod` binary being specified using `$LMOD_CMD`, but `spider` not being available in `$PATH`, which manifests itself in EasyBuild v1.11.0 as follows:

```
Traceback (most recent call last):
  File "test/framework/modulestool.py", line 141, in test_lmod_specific
    self.assertTrue(lmod.update(fake=True), "Updated local Lmod spider cache is non-empty")
  File "/Users/kehoste/work/easybuild-framework/easybuild/tools/modules.py", line 653, in update
    proc = subprocess.Popen(cmd, stdout=PIPE, stderr=PIPE, env=os.environ)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 672, in __init__
    errread, errwrite)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1202, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory
```