test: drop JET prerelease gating

[Beforerr]

JET resolves on current prerelease Julia (and pre CI is allowed to fail), so the runtime Pkg.add("JET") gating is unnecessary. JET is a normal test dep again; unused Pkg test dep removed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	main	a805644...	main / a805644...
elx/full_load	0.0364 ± 0.0022 ms	0.0364 ± 0.0025 ms	1 ± 0.092
elx/sum_array	2.62 ± 1.2 μs	2.98 ± 1.4 μs	0.876 ± 0.57
elx/sum_lazy	2.8 ± 0.46 μs	2.91 ± 0.52 μs	0.962 ± 0.23
elx/sum_var_access	2.12 ± 1.1 μs	2.02 ± 1.1 μs	1.05 ± 0.76
elx/var_access	0.251 ± 0.011 μs	0.241 ± 0.01 μs	1.04 ± 0.063
mms/full_load	21.6 ± 2.2 μs	21.4 ± 2.2 μs	1.01 ± 0.15
mms/sum_array	0.0955 ± 0.0021 s	0.0953 ± 0.0022 s	1 ± 0.032
mms/sum_slice	3.52 ± 0.053 ms	3.51 ± 0.083 ms	1 ± 0.028
mms/sum_var_access	3.53 ± 0.092 ms	3.5 ± 0.05 ms	1.01 ± 0.03
mms/var_access	0.09 ± 0.01 μs	0.09 ± 0 μs	1 ± 0.11
time_to_load	0.166 ± 0.002 s	0.165 ± 0.0025 s	1.01 ± 0.02

Memory benchmarks

	main	a805644...	main / a805644...
elx/full_load	0.217 k allocs: 24.8 kB	0.217 k allocs: 24.8 kB	1
elx/sum_array	12 allocs: 28 kB	12 allocs: 28 kB	1
elx/sum_lazy	12 allocs: 28 kB	12 allocs: 28 kB	1
elx/sum_var_access	14 allocs: 28.3 kB	14 allocs: 28.3 kB	1
elx/var_access	1 allocs: 0.234 kB	1 allocs: 0.234 kB	1
mms/full_load	0.065 k allocs: 6.45 kB	0.065 k allocs: 6.45 kB	1
mms/sum_array	1.04 k allocs: 31.7 MB	1.04 k allocs: 31.7 MB	1
mms/sum_slice	0.069 k allocs: 1.29 MB	0.069 k allocs: 1.29 MB	1
mms/sum_var_access	0.069 k allocs: 1.29 MB	0.069 k allocs: 1.29 MB	1
mms/var_access	1 allocs: 0.219 kB	1 allocs: 0.219 kB	1
time_to_load	0.2 k allocs: 11.8 kB	0.2 k allocs: 11.8 kB	1