Improved overflow support: Parameterized torture tests

[sungshik]

Primary goal of this PR

This PR parameterizes the torture tests with auto-handlers for OVERFLOW events, so we now automatically run all torture tests with MemorylessRescanner (i.e., OnOverflow.ALL) and IndexingRescanner (i.e., OnOverflow.DIRTY).

As JUnit doesn't seem to have easy support for combining @ParameterizedTest and @RepeatedTest, but we do need both, three approaches were considered:

• Implement our own test template that combines them, as demonstrated here
• Use containers
• Provide a stream of repeated input values to instantiate the test parameter

This PR implements the third approach because: (a) it's conceptually easy; (b) it yields the least amount of extra code; (c) as a result, it seems to strike the best balance for now between effort vs. number of torture tests that need it (only one). If, at some point in the future, we happen to have many more (torture) tests that need it, then maybe we should revisit and invest some effort to use one of the other approaches.

Other changes

This PR also fixes two issues in IndexingRescanner that were revealed by the parameterized tests, namely: (a) a small fix that makes sure a file really doesn't exist when a DELETED event is generated; (b) a larger fix that adds more hierarchical structure to the index map---two levels instead of one level---to address performance issues.

[codecov]

Codecov Report

Attention: Patch coverage is 61.90476% with 16 lines in your changes missing coverage. Please review.

Project coverage is 80.4%. Comparing base (fd68764) to head (23506ae).
Report is 13 commits behind head on improved-overflow-support-main.

Files with missing lines	Patch %	Lines
...g/swat/watch/impl/overflows/IndexingRescanner.java	61.9%	10 Missing and 6 partials ⚠️

Additional details and impacted files

@@ Coverage Diff @@## improved-overflow-support-main #29 +/- ##
==================================================================
- Coverage 81.5% 80.4% -1.2% 
Complexity 130 130 ==================================================================
Files 16 16 Lines 532 567 +35 Branches 54 64 +10 ==================================================================
+ Hits 434 456 +22 - Misses 62 70 +8 - Partials 36 41 +5 

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