[MOD-15026] allow passing startup grace

[JoanFM]

Why this change

RLTest's StandardEnv.startEnv has a hardcoded time.sleep(0.1) between subprocess.Popen(redis-server ...) and the liveness probe (_isAlive + waitForRedisToStart → wait_for_conn). That 100 ms grace period is the only window during which the framework tolerates a redis-server that exits on its own (e.g. because a loadmodule argument was rejected and the server aborted).

For tests that intentionally pass invalid module args and expect the server to fail to start, this is racy:

• If redis-server aborts within 100 ms → _isAlive returns False, masterProcess is set to None, Env.__init__ returns cleanly, and the test can assert not env.isUp().
• If redis-server aborts slightly later (common under load, ASAN, macOS, CI) → _isAlive returns True, the framework opens a connection, and wait_for_conn eventually raises Exception('Redis server is dead ...'). That exception is raised from inside Env.__init__ before RTestInstance.currEnv is assigned, so even when the test's own try/except swallows it, the runner reports a spurious <Environment destroyed> failure at teardown.

Concretely, this has been making testconfigMultiTextOffsetDeltaSlopNeg flaky in RediSearch, where we pass MULTI_TEXT_SLOP -1 on purpose and expect the module to reject it during init.

What this PR does

Adds a startupGraceSecs parameter (default 0.1, i.e. identical to the current behavior) to StandardEnv.__init__ and Env.__init__, plumbed through getEnvKwargs, and replaces the hardcoded time.sleep(0.1) in startEnv with time.sleep(self.startupGraceSecs).

Tests that expect the server to abort during module init can then bump the grace period, e.g.:

env = Env(moduleArgs='MULTI_TEXT_SLOP -1', startupGraceSecs=2)
assert not env.isUp()

This keeps the liveness probe from racing with the abort without affecting any existing caller.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.26%. Comparing base (02cfbb9) to head (e6148b6).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
+ Coverage   32.46%   34.26%   +1.79%     
==========================================
  Files          17       17              
  Lines        2597     2697     +100     
==========================================
+ Hits          843      924      +81     
- Misses       1754     1773      +19     

Flag	Coverage Δ
unittests	34.26% <100.00%> (+1.79%)	⬆️

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