Fix: Correct Scope of Symbolic Variable in `where` Rule Test

[bowenszhu]

The original test for the where clause in rules incorrectly handled the scope of the symbolic variable a. Two distinct symbolic variables with the same name were created, leading to unexpected behavior when checking for object equality in the _f function.

This PR moves the definition of _f inside the @testset block, ensuring that it references the same symbolic variable a as the rule definition. This correction aligns the test with the intended behavior of the where clause.

[github-actions[bot]]

Benchmark Results

	master	fc7509587c2550...	master/fc7509587c2550...
overhead/acrule/a+2	0.768 ± 0.016 μs	0.726 ± 0.02 μs	1.06
overhead/acrule/a+2+b	0.735 ± 0.013 μs	0.703 ± 0.019 μs	1.04
overhead/acrule/a+b	0.254 ± 0.0089 μs	0.253 ± 0.012 μs	1
overhead/acrule/noop:Int	25.9 ± 0.05 ns	25.3 ± 0.05 ns	1.02
overhead/acrule/noop:Sym	0.0366 ± 0.0056 μs	0.0365 ± 0.0053 μs	1
overhead/rule/noop:Int	0.0438 ± 0.0011 μs	0.0451 ± 0.0011 μs	0.97
overhead/rule/noop:Sym	0.055 ± 0.0033 μs	0.0588 ± 0.0023 μs	0.936
overhead/rule/noop:Term	0.0547 ± 0.003 μs	0.0585 ± 0.0021 μs	0.935
overhead/ruleset/noop:Int	0.126 ± 0.0032 μs	0.127 ± 0.0031 μs	0.991
overhead/ruleset/noop:Sym	0.147 ± 0.0044 μs	0.149 ± 0.0053 μs	0.987
overhead/ruleset/noop:Term	3.02 ± 0.1 μs	3.06 ± 0.11 μs	0.987
overhead/simplify/noop:Int	0.167 ± 0.0012 μs	0.149 ± 0.0021 μs	1.12
overhead/simplify/noop:Sym	0.184 ± 0.0077 μs	0.17 ± 0.0041 μs	1.08
overhead/simplify/noop:Term	0.0356 ± 0.0016 ms	0.0348 ± 0.0016 ms	1.02
overhead/simplify/randterm (+, *):serial	0.0848 ± 0.001 s	0.0851 ± 0.00089 s	0.997
overhead/simplify/randterm (+, *):thread	0.051 ± 0.03 s	0.0509 ± 0.029 s	1
overhead/simplify/randterm (/, *):serial	0.212 ± 0.0076 ms	0.205 ± 0.007 ms	1.03
overhead/simplify/randterm (/, *):thread	0.239 ± 0.0091 ms	0.234 ± 0.0084 ms	1.02
overhead/substitute/a	0.0588 ± 0.0015 ms	0.0603 ± 0.0017 ms	0.975
overhead/substitute/a,b	0.052 ± 0.0015 ms	0.0521 ± 0.0014 ms	0.997
overhead/substitute/a,b,c	16.7 ± 0.6 μs	17.2 ± 0.6 μs	0.97
polyform/easy_iszero	28.6 ± 1.9 μs	28.8 ± 2.1 μs	0.993
polyform/isone	3.1 ± 0.01 ns	2.79 ± 0.01 ns	1.11
polyform/iszero	1.1 ± 0.029 ms	1.09 ± 0.028 ms	1.01
polyform/simplify_fractions	1.61 ± 0.04 ms	1.6 ± 0.04 ms	1.01
time_to_load	2.11 ± 0.0054 s	2.12 ± 0.013 s	0.999

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR. Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).