add assertion to compare to file content.

src/main/kotlin/ltd/hlaeja/test/Assertion.kt

@@ -0,0 +1,15 @@
+package ltd.hlaeja.test
+
+import org.assertj.core.api.AbstractStringAssert
+
+@Suppress("unused")
+fun <SELF : AbstractStringAssert<SELF>?> AbstractStringAssert<SELF>.compareToFile(
+    file: String,
+): AbstractStringAssert<SELF> = this::class.java.classLoader
+    .getResourceAsStream(file)
+    ?.bufferedReader()
+    ?.readText()
+    ?.let { this.isEqualTo(it) }
+    ?: throw UnsupportedOperationException(
+        "Attempted to compare assertion object to context of a file but expected file was not found: $file",
+    )