test(fix): refactor SQL query assertions in SpannerStatementQueryExecutor.getChildrenRowsQuery to prevent flakiness

[williamlin0825]

Related to issue: #3290

A test in the API getChildrenRowsQueryTest checks for the result from getSQL() from SpannerStatementQueryExecutor.getChildrenRowsQuery. However, it checks the result in a specific order, which is not guaranteed. Here is the affected test:

spring-cloud-gcp/spring-cloud-gcp-data-spanner/src/test/java/com/google/cloud/spring/data/spanner/repository/query/SpannerQueryLookupStrategyTests.java

Lines 176 to 179 in 05ad3ef

	assertThat(statement.getSql())
	.isEqualTo(
	"SELECT deleted, id3, id, id_2 FROM child_test_table WHERE ((id = @tag0 AND id_2 ="
	+ " @tag1)) AND (deleted = false)");

This PR propose to simply check whether the selected field names are correct instead of checking in a specific order.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.