Add unit tests for `isValidDomain`

[jackwotherspoon]

The regex used to validate domain names in this library is not able to compile in
languages like Go due to lookahead assertions. As such, we should add unit tests
around isValidDomain to test common test cases and domain names.

cloud-sql-jdbc-socket-factory/core/src/main/java/com/google/cloud/sql/core/CloudSqlInstanceName.java

Line 68 in 595e620

public static boolean isValidDomain(String domain) {

This will allow for differing regex to have a baseline of tested common paths.