Keep LdapKerberosImpalaShellTest* compatible with older guava versions

LdapKerberosImpalaShellTestBase.java and LdapKerberosImpalaShellTest.java use the ImmutableMap.of function with 8+ pairs. Older versions of guava like 28.1-jre do not have ImmutableMap.of() for that number of arguments.

Since we often want to use the guava version that the underlying Hadoop/Hive use, it can be useful for compatibility to be able to build against older guava (like 28.1-jre).

Most other code is fine, so if we switch these locations to use ImmutableMap.<String, String>builder(), then the whole codebase can compile with the older guava (while remaining forward compatible as well).