Use SUPPORTS_REMOVE for CollectionRemoveIfTester feature requirements.

[Divyansh151005]

Problem

CollectionRemoveIfTester was inconsistent about which collection feature flags gate removeIf tests:

• Positive removeIf tests required SUPPORTS_ITERATOR_REMOVE.
• Unsupported-case tests correctly used absent = SUPPORTS_REMOVE.

Collections that support removeIf (and remove) but not iterator removal therefore skipped all removeIf coverage in Guava Testlib.

Fixes #6076

Solution

Require SUPPORTS_REMOVE for the positive removeIf tests, matching CollectionRemoveTester and the unsupported tests in the same class.

Add CollectionRemoveIfFeatureTest, which builds a suite for a collection with SUPPORTS_REMOVE but without SUPPORTS_ITERATOR_REMOVE and verifies that removeIf tests are included.

Testing

$ export JAVA_HOME=~/.m2/jdks/jdk-26.0.2+10/Contents/Home
$ ./mvnw -B -Dtoolchain.skip -P!standard-with-extra-repos test \
    -Dmaven.javadoc.skip=true -Dsurefire.toolchain.version=17 \
    -Dtest=CollectionRemoveIfFeatureTest -DfailIfNoTests=false -pl guava-testlib

[INFO] Running com.google.common.collect.testing.CollectionRemoveIfFeatureTest
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] BUILD SUCCESS

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