Unit test real galaxies

[ismael-mendoza]

Closes #470

Useful references:

• https://galsim-developers.github.io/GalSim/_build/html/real_gal.html#galsim.GalaxySample.makeGalaxy
• https://github.com/GalSim-developers/GalSim/blob/dfb319cdec49d769ce85b2ccaeeccf242824836d/examples/demo11.py#L93

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 63.90%. Comparing base (3eebcbd) to head (40dcdee).

Files	Patch %	Lines
btk/draw_blends.py	85.71%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #485 +/- ## ========================================== + Coverage 61.69% 63.90% +2.20% ========================================== Files 19 19 Lines 1569 1568 -1 ========================================== + Hits 968 1002 +34 + Misses 601 566 -35 ``` | [Flag](https://app.codecov.io/gh/LSSTDESC/BlendingToolKit/pull/485/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/LSSTDESC/BlendingToolKit/pull/485/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC) | `63.90% <85.71%> (+2.20%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#carryforward-flags-in-the-pull-request-comment) to find out more.

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