Fix mismatching Parameter initial field values

[Marcono1234]

If I understand it correctly, if a Mojo parameter has a @Parameter#defaultValue, that value overwrites the initial value of the field (if any).

This pull request therefore fixes two cases where the initial field value did not match the @Parameter#defaultValue, which could have lead to confusion.

• For AbstractFromDependenciesMojo#failOnMissingClassifierArtifact it seems this issue was accidentally introduced by 1592cdc#diff-b352be9b15d48a15ddf79e115c28c4cc776b7ec2662f553e189a978261d34881R100; the Javadoc had previously default-value="true", which was matching the field value true, and was accidentally changed to false.
  Though I am not going to adjust the default value to be true again since it has been false now for multiple years apparently.

  The test classes TestUnpackDependenciesMojo and TestCopyDependenciesMojo had to be adjusted because they were apparently not creating the Mojo instances they way Maven would normally do and therefore were relying on the field values instead of the defaultValue. For example when using the following pom.xml and running mvn package, you will see that the Dependency Plugin does not report an error for a missing classifier by default:

  pom.xml (click to expand)

  <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    
    <groupId>com.mycompany.app</groupId>
    <artifactId>my-app</artifactId>
    <version>1.0-SNAPSHOT</version>
    
    <dependencies>
      <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.12</version>
      </dependency>
    </dependencies>
  
    <build>
      <plugins>
        <plugin>
          <groupId>org.apache.maven.plugins</groupId>
          <artifactId>maven-dependency-plugin</artifactId>
          <version>3.7.1</version>
          <executions>
            <execution>
              <id>copy-dependencies</id>
              <phase>package</phase>
              <goals>
                <goal>copy-dependencies</goal>
              </goals>
              <configuration>
                <classifier>does-not-exist</classifier>
                <outputDirectory>${project.build.directory}/dependencies</outputDirectory>
              </configuration>
            </execution>
          </executions>
        </plugin>
      </plugins>
    </build>
  </project>

• For CopyDependenciesMojo#copyPom it looks like it always had this mismatch since it was added by 32a29e6

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[Marcono1234]

Removed the parameters from this test method because it looks like they were unused since 809bfe1, so testArtifactNotFound and testArtifactResolutionException were effectively equivalent.