Fix code example formatting in best-practices article

Signed-off-by: Lajos Papp <lajos.papp@sequenceiq.com>
Fix quoting of TESTFLAGS in devenvironment.md

Copy and pasting the example should now work, rather than TESTFLAGS being silently ignored.
Merge pull request #8212 from hugoduncan/patch-1

Fix quoting of TESTFLAGS in devenvironment.md
Merge pull request #8205 from sequenceiq/master

Fix code example formatting in best-practices article