[#28579] yb-admin: Add AdminCli.TestFlushTable* unit tests

[kenju]

#28579

Changes Made

Added a couple of new test cases for yb-admin flush_table commands in src/yb/tools/yb-admin-test.cc.

Example use cases:

# with namespace and table name
./bin/yb-admin flush_table flush_table ysql.yugabyte users 30

# with tableid
./bin/yb-admin flush_table tableid.000034d400003000800000000000431edd 30

Confirmed that those pass locally:

$ ./yb_build.sh release --cxx-test yb-admin-test --gtest_filter="AdminCliTest.TestFlushTable*"

1/1 Test #323: tools_yb-admin-test ..............   Passed    9.44 sec

The following tests passed:
	tools_yb-admin-test

100% tests passed, 0 tests failed out of 1

Total Test time (real) =   9.46 sec