Control the number of rows in chunks returned by LimitExec

[qw4990]

What problem does this PR solve?

Control the number of rows in chunks returned by LimitExec.
Following up #9293, this PR is a subtask of #9166.

What is changed and how it works?

• Introduce adjustRequiredRows into LimitExec to adjust the required rows before each call of
  child's Next.
• Introduce requiredRowsDataSource as mock DataSource when test.
• Add some unit tests for LimitExec.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #9354 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9354      +/-   ##
==========================================
+ Coverage   67.05%   67.07%   +0.02%     
==========================================
  Files         372      372              
  Lines       78043    78053      +10     
==========================================
+ Hits        52332    52357      +25     
+ Misses      21033    21023      -10     
+ Partials     4678     4673       -5

Impacted Files	Coverage Δ
executor/executor.go	67.89% <100%> (+0.86%)	⬆️
store/tikv/lock_resolver.go	41.7% <0%> (-0.95%)	⬇️
ddl/delete_range.go	79.36% <0%> (+5.82%)	⬆️
ddl/session_pool.go	93.1% <0%> (+10.34%)	⬆️

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[zz-jason]

LGTM

[qw4990]

PTAL~ @lysu

[qw4990]

/run-all-tests

[qw4990]

/run-all-tests

[lysu]

LGTM