Tests for solve_interms_ofvar and bug fixes

[n0rbed]

Fixed multiplicity bugs by adding correct checking in solve_univar and added some tests for solve_interms_ofvar. One thing to note; solve_interms_ofvar doesnt work with parameters, so say the equations provided are one less than the variables present in the system, it wont get all the other variables in terms of one variable. E.g.:

julia> eq = (a*x+b)*s^2 - 2s^2 + 2*b*s - 3*s

julia> Symbolics.solve_interms_ofvar(eq, s)
┌ Warning: Infinite number of solutions
└ @ SymbolicsGroebnerExt ~/code/julia/Symbolics.jl/ext/SymbolicsGroebnerExt.jl:228
ERROR: AssertionError: Not a polynomial

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.55%. Comparing base (3e31a75) to head (62d8d0e).
Report is 29 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1345       +/-   ##
===========================================
+ Coverage    6.69%   78.55%   +71.86%     
===========================================
  Files          47       49        +2     
  Lines        4618     4813      +195     
===========================================
+ Hits          309     3781     +3472     
+ Misses       4309     1032     -3277     

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