default_variants is ignored when using scons --test

[feedhandler]

In my SConstruct I have default_variants = ['dbg',], but this seems to be ignored when I build with scons --test
normal build, it works OK and builds just the dbg variant:

dmsdev@dmsdev-VirtualBox> scons
scons: Reading SConscript files ...
cuppa: configure: [info] No settings to load, skipping configure
cuppa: version: [info] cuppa: version 0.9.26
^Ccuppa: construct: [info] using sconstruct file [SConstruct]
cuppa: construct: [info] available toolchains are ['gcc54', 'gcc5', 'gcc', 'clang38', 'clang']
cuppa: construct: [info] default profiles are ['']
cuppa: construct: [info] default dependencies are ['spdlog']
cuppa: construct: [info] Using sub-sconscripts ['./src/SConscript']
cuppa: construct: [info] Default build variants of ['dbg'] being used.
scons: done reading SConscript files.
scons: Building targets ...
Progress( SconstructBegin )
Progress( Begin sconscript: [./src/SConscript] )
Progress( Starting variant: [_build/src/gcc54/dbg/x86_64/c++1z] )
Progress( Finished variant: [_build/src/gcc54/dbg/x86_64/c++1z] )
Progress( End sconscript: [./src/SConscript] )
Progress( SconstructEnd )
scons: done building targets.
dmsdev@dmsdev-VirtualBox> 

build with --test, it builds both dbg and rel variants:

dmsdev@dmsdev-VirtualBox> scons --test
scons: Reading SConscript files ...
cuppa: configure: [info] No settings to load, skipping configure
cuppa: version: [info] cuppa: version 0.9.26
^Ccuppa: construct: [info] using sconstruct file [SConstruct]
cuppa: construct: [info] available toolchains are ['gcc54', 'gcc5', 'gcc', 'clang38', 'clang']
cuppa: construct: [info] default profiles are ['']
cuppa: construct: [info] default dependencies are ['spdlog']
cuppa: construct: [info] Using sub-sconscripts ['./src/SConscript']
cuppa: construct: [info] No active variants specified so toolchain defaults of ['rel', 'dbg'] being used.
scons: done reading SConscript files.
scons: Building targets ...
Progress( SconstructBegin )
Progress( Begin sconscript: [./src/SConscript] )
Progress( Starting variant: [_build/src/gcc54/rel/x86_64/c++1z] )
Progress( Finished variant: [_build/src/gcc54/rel/x86_64/c++1z] )
Progress( Starting variant: [_build/src/gcc54/dbg/x86_64/c++1z] )
Progress( Finished variant: [_build/src/gcc54/dbg/x86_64/c++1z] )
Progress( End sconscript: [./src/SConscript] )
Progress( SconstructEnd )
scons: done building targets.
dmsdev@dmsdev-VirtualBox>