cmd/evm: speed up TestEvmRun

[lupin012]

Closes #15153

Problem: TestEvmRun took ~2.9s. Two of its four sub-cases pass --bench, which calls testing.Benchmark: it loops until it accumulates ~1s of measured time (the default benchtime), no matter how trivial the executed bytecode is. The default cannot be overridden from the command line, because the re-exec'd child parses flags with urfave/cli, not with the testing package.

Two changes (test code only):

1. The re-exec handler in TestMain sets -test.benchtime from the EVM_TEST_BENCHTIME env variable. It is opt-in: without the variable the 1s default stays. TestEvmRun passes 1x (a single measured iteration, deterministic) per child via a new optional Env field on cmdtest.TestCmd. 2.94s → 0.80s.
2. The four sub-cases now run as named parallel subtests, so the child processes overlap. 0.80s → 0.22s (stable across 10 runs, race detector clean, full cmd/evm package still passes).

Question for reviewers: at 0.22s the test is well below the 500ms threshold that motivated its exclusion from short mode in #15144. Can we also remove the testing.Short() skip and bring it back into the short suite?