broken foregroundlock in codegentest.cpp

[floris]

codegentest failed because the foregroundlock was not(probably never)
taken by the foreground thread than at some point it tried to
temporarily release exactely this lock(in duchain.cpp), failing
miserably. this patch makes the duchain.cpp function take the lock right
before it tries to temporarily release if it hasn't already got it. it
makes the test run, but is more of a hack than a fix...

Floris