Fix typos in CCFLAGS test

[mwichmann]

Trivial: in two DefaultEnvironment calls, tool=[] -> tools=[]. Doesn't affect the running of the tests, just enables the speedup as was intended.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation