RFR: JDK-8300909: Update com/sun/jndi/dns/Test6991580.java manual test instruction

[Bill Huang]

Test failed for exception in thread "main" java.lang.IllegalAccessError: class IPv6NameserverPlatformParsingTest (in unnamed module @0x72a8ab1a) cannot access class com.sun.jndi.dns.DnsContextFactory (in module jdk.naming.dns) because module jdk.naming.dns does not export com.sun.jndi.dns to unnamed module @0x72a8ab1a
	at IPv6NameserverPlatformParsingTest.main(IPv6NameserverPlatformParsingTest.java:46)

This task is created to update test instruction to include --add-modules and --add-opens jvm parameters to java command.

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Commit messages:
 - Update com/sun/jndi/dns/Test6991580.java manual test instruction

Changes: https://git.openjdk.org/jdk/pull/12351/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12351&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300909
  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12351.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12351/head:pull/12351

PR: https://git.openjdk.org/jdk/pull/12351