Fix rmm_mode=managed parameter for gtests

[davidwendt]

When using parameter --rmm_mode=managed for gtests Invalid RMM allocation mode: managed exception is thrown.
The logic in include/cudf_test/base_fixture.hpp is just missing a return statement.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[davidwendt]

rerun tests

[codecov]

Codecov Report

Merging #6912 (a155448) into branch-0.18 (e9aedb2) will increase coverage by 0.03%.
The diff coverage is 83.05%.

@@               Coverage Diff               @@
##           branch-0.18    #6912      +/-   ##
===============================================
+ Coverage        81.94%   81.97%   +0.03%     
===============================================
  Files               96       96              
  Lines            16164    16230      +66     
===============================================
+ Hits             13246    13305      +59     
- Misses            2918     2925       +7     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/string.py	86.30% <66.66%> (-0.53%)	⬇️
python/cudf/cudf/io/avro.py	78.57% <66.66%> (-3.25%)	⬇️
python/cudf/cudf/io/csv.py	94.00% <66.66%> (-1.75%)	⬇️
python/cudf/cudf/io/json.py	96.55% <66.66%> (-3.45%)	⬇️
python/cudf/cudf/io/orc.py	88.40% <66.66%> (-0.99%)	⬇️
python/cudf/cudf/utils/ioutils.py	78.71% <80.00%> (-1.18%)	⬇️
python/cudf/cudf/core/column/timedelta.py	89.53% <87.50%> (+0.04%)	⬆️
python/cudf/cudf/utils/utils.py	85.35% <94.11%> (+1.10%)	⬆️
python/cudf/cudf/core/column/categorical.py	93.37% <100.00%> (+0.03%)	⬆️
python/cudf/cudf/core/column/datetime.py	88.94% <100.00%> (ø)
... and 11 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8c8e05f...a155448. Read the comment docs.