only add .mini tests if dune-testtools is present

[ftalbrecht]

merge when ready

[codecov[bot]]

Codecov Report

Merging #201 into master will decrease coverage by 2.66%. The diff coverage is n/a.

Impacted Files	Coverage Δ
dune/xt/common/parallel/threadmanager.cc	90.32% <0%> (-9.68%)	:arrow_down:
dune/xt/common/validation.hh	78.46% <0%> (-9.07%)	:arrow_down:
dune/xt/common/float_cmp_internal.hh	88.03% <0%> (-5%)	:arrow_down:
dune/xt/common/configuration.cc	42.29% <0%> (-3.09%)	:arrow_down:
dune/xt/common/configuration.hh	67.08% <0%> (-0.57%)	:arrow_down:
dune/xt/common/string_internal.hh	73.09% <0%> (-0.17%)	:arrow_down:
dune/xt/common/float_cmp.hh	100% <0%> (ø)	:arrow_up:
dune/xt/common/parallel/threadstorage.hh	100% <0%> (ø)	:arrow_up:
dune/xt/common/vector_statistics.hh
dune/xt/common/fix-ambiguous-std-math-overloads.cc
... and 4 more