fix(CellBudgetFile): strip auxname for imeth 5

[mwtoews]

There are two parts to this PR (each are both a commit):

• A one-line fix (on L1781) to CellBudgetFile to strip spaces from auxname (e.g.) from "IFACE " to "IFACE". This was already considered for imeth 6 and most (all?) other 16-character strings from binary files.
• Move tests related to binary files from test_formattedfile.py to test_binaryfile.py without any other changes, except for testing expected auxname values, which can be viewed in the other commit.

[codecov]

Codecov Report

Merging #1716 (5a69906) into develop (2552de5) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           develop   #1716   +/-   ##
=======================================
  Coverage     71.9%   71.9%           
=======================================
  Files          253     253           
  Lines        55891   55891           
=======================================
+ Hits         40202   40212   +10     
+ Misses       15689   15679   -10     

Impacted Files	Coverage Δ
flopy/utils/binaryfile.py	80.1% <100.0%> (ø)
flopy/utils/get_modflow.py	65.3% <0.0%> (+2.4%)	⬆️