tests: simplify `random_fe_non_zero` (remove loop limit and unneeded normalize) #57

Job	Run time
x64 (MSVC): C++ (public headers)	34s
x64 (MSVC): Windows (VS 2022, shared)	7m 24s
x64 (MSVC): Windows (VS 2022, static)	4m 41s
x64 (MSVC): Windows (VS 2022, int128_struct)	7m 6s
x64 (MSVC): Windows (VS 2022, int128_struct with __(u)mulh)	7m 7s
x86 (MSVC): Windows (VS 2022)	7m 25s
SageMath prover	1m 3s
x86_64: macOS Ventura (int64, yes, yes, yes, yes)	5m 31s
x86_64: macOS Ventura (int64, yes, yes, yes, yes, gcc)	6m 19s
x86_64: macOS Ventura (int128_struct, 2, 4)	3m 55s
x86_64: macOS Ventura (int128, yes, yes, yes)	4m 2s
x86_64: macOS Ventura (int128, yes, yes)	2m 50s
x86_64: macOS Ventura (int128, yes, yes, yes, yes, gcc)	4m 31s
x86_64: macOS Ventura (int128, yes, yes, yes, yes, -DVERIFY)	3m 50s
x86_64: macOS Ventura (distcheck)	4m 2s
	1h 10m 20s

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