Fix regression for newline separators in SQL scripts

Changes made in conjunction with SPR-9531, introduced a regression with
regard to support for using a single newline character as the statement
separator within SQL scripts. Investigation of the cause of this issue
resulted in the discovery of another, similar issue: support for
multiple newlines as a statement separator has been broken for years
but has gone unnoticed until now.

The reason that both of these issues have gone unnoticed is a result of
the fact that the test suite only executes SQL script integration tests
against HSQL DB, and HSQL does not care if two statements occur on the
same line; whereas, the H2 database will throw an exception if multiple
statements are included on the same line when executing an update.

This commit addresses both of these issues and provides further
enhancements to Spring's SQL script support as follows.

 - ScriptUtils now properly checks if the supplied script contains the
   custom statement separator or default separator before falling back
   to the 'fallback' separator (i.e., newline).

 - Introduced FALLBACK_STATEMENT_SEPARATOR constant in ScriptUtils.

 - ScriptUtils.readScript() no longer omits empty lines from the input
   file since a statement separator string may in fact be composed of
   multiple newline characters.

 - Introduced overloaded variants of splitSqlScript() and
   executeSqlScript() in ScriptUtils with smaller argument lists for
   common use cases.

 - Extracted AbstractDatabasePopulatorTests from DatabasePopulatorTests
   and introduced concrete HsqlDatabasePopulatorTests and
   H2DatabasePopulatorTests subclasses for testing against HSQL and H2.

 - Split ScriptUtilsTests into ScriptUtilsUnitTests and
   ScriptUtilsIntegrationTests for faster builds.

Issue: SPR-11560