Fix is_power for e.g. ZZPolyRing

[fingolfin]

Without this, we get in Nemo:

julia> ZZx, x = ZZ[:x];  is_power(2^2 * x^2, 2)
(true, 4*x)

With it, the correct result is produced:

julia> ZZx, x = ZZ[:x];  is_power(2^2 * x^2, 2)
(true, 2*x)

Motivated by #2051 which also contains some tests for this, which requires implementing factorization for some rings in AA... I did not want to go to these lengths, so i run the above and a couple other tests in Nemo... That is:

ZZx, x = ZZ[:x]
#QQx, x = QQ[:x]

@test is_power(x, 2) == (false, x)
@test is_power(x, 3) == (false, x)
@test is_power(x^2, 2) == (true, x)
@test is_power(3 * x^2, 2) == (false, 3 * x^2)
@test is_power(2^2 * x^2, 2) == (true, 2 * x)
@test is_power(2^3 * x^3, 3) == (true, 2 * x)
@test is_power(3^4 * x^4, 2) == (true, 9 * x^2)
@test is_power(-3^4 * x^4, 2) == (false, -81 * x^4)
@test is_power(-3^3 * x^3, 3) == (true, -3 * x)
@test is_power(3^2 * (x + 1)^2 * x^2, 2) == (true, 3 * (x + 1) * x)
@test is_power(-3^2 * (x + 1)^2 * x^2, 2) == (false, -3^2 * (x + 1)^2 * x^2)

Once with the ring over ZZ and once with the one over QQ.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.00%. Comparing base (bee81db) to head (aed9edf).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
src/generic/Misc/Poly.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2241   +/-   ##
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  Coverage   88.00%   88.00%           
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  Files         127      127           
  Lines       31803    31803           
=======================================
  Hits        27989    27989           
  Misses       3814     3814           

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[SoongNoonien]

@fingolfin and I talked about this and this should be correct now.