Refactor cargo test Gradle steps

[crisidev]

In PR #1366 we noticed we are replicating the cargo gradle testing tasks in codegen-server/build.gradles.kt, codegen-server/python/build.gradle.kt and codegen/build.gradle.kt.

It will be good to refactor and make the steps reusable and define them in a single place.

Tasks to refactor:

task("generateSmithyBuild") {
    description = "generate smithy-build.json"
    doFirst {
        projectDir.resolve("smithy-build.json")
            .writeText(
                generateSmithyBuild(
                    rootProject.projectDir.absolutePath,
                    pluginName,
                    codegenTests(properties, allCodegenTests)
                )
            )
    }
}

task("generateCargoWorkspace") {
    description = "generate Cargo.toml workspace file"
    doFirst {
        buildDir.resolve("$workingDirUnderBuildDir/Cargo.toml")
            .writeText(generateCargoWorkspace(pluginName, codegenTests(properties, allCodegenTests)))
    }
}

tasks["smithyBuildJar"].dependsOn("generateSmithyBuild")

tasks["assemble"].finalizedBy("generateCargoWorkspace")

tasks.register<Exec>(Cargo.CHECK.toString) {
    workingDir("$buildDir/$workingDirUnderBuildDir")
    environment("RUSTFLAGS", defaultRustFlags)
    commandLine("cargo", "check")
    dependsOn("assemble")
}

tasks.register<Exec>(Cargo.TEST.toString) {
    workingDir("$buildDir/$workingDirUnderBuildDir")
    environment("RUSTFLAGS", defaultRustFlags)
    commandLine("cargo", "test")
    dependsOn("assemble")
}

tasks.register<Exec>(Cargo.DOCS.toString) {
    workingDir("$buildDir/$workingDirUnderBuildDir")
    environment("RUSTDOCFLAGS", defaultRustDocFlags)
    commandLine("cargo", "doc", "--no-deps")
    dependsOn("assemble")
}

tasks.register<Exec>(Cargo.CLIPPY.toString) {
    workingDir("$buildDir/$workingDirUnderBuildDir")
    environment("RUSTFLAGS", defaultRustFlags)
    commandLine("cargo", "clippy")
    dependsOn("assemble")
}

tasks["test"].finalizedBy(cargoCommands(properties).map { it.toString })

tasks["clean"].doFirst { delete("smithy-build.json") }