Add unit tests for FilterValve, ProxyErrorReportValve and SemaphoreValve

[onkar717]

Summary

This PR improves test coverage by adding unit tests for several Valve
implementations that currently lack dedicated tests:

• FilterValve
• ProxyErrorReportValve
• SemaphoreValve

Motivation

While reviewing Tomcat valve implementations, these components were
identified as having little or no direct test coverage. Adding tests
helps improve reliability and prevents regressions during future
changes.

Changes

Added new test classes:

• TestFilterValve
• TestProxyErrorReportValve
• TestSemaphoreValve

The tests cover:

• Basic initialization behavior
• Request/response processing
• Error handling paths
• Concurrency behavior (SemaphoreValve)
• Configuration edge cases

Testing

All tests pass locally using:

Run all 3 new test classes

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64 ant test \
  -Dtest.name=**/TestProxyErrorReportValve.java,**/TestFilterValve.java,**/TestSemaphoreValve.java

Run with coverage to verify coverage increase

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64 ant test -Dtest.coverage=true \
  -Dtest.haltonfailure=false \
  -Dtest.name=**/valves/Test*.java

No existing tests were modified.

Impact

No functional changes.
Test coverage improvement only.