Entry point assertion class for custom assertions

Currently, `assertThat` methods from custom asserts(`SpanAssert`,
`SpansAssert`, `TracerAssert`) do not have an entry point assertion
class. Hence, users need to static import individual assert classes.

This change introduces an entry point assertion class. Users now simply
static import this class and all `assertThat` methods are available for
use.

micrometer-tracing-tests/micrometer-tracing-test/src/main/java/io/micrometer/tracing/test/simple/TracingAssertions.java

@@ -0,0 +1,28 @@
+package io.micrometer.tracing.test.simple;
+
+import java.util.Collection;
+
+import io.micrometer.tracing.exporter.FinishedSpan;
+
+/**
+ * Entry point assertions.
+ *
+ * @author Tadaya Tsuyukubo
+ * @since 1.0.0
+ */
+public class TracingAssertions {
+
+    @SuppressWarnings("rawtypes")
+    public static SpanAssert assertThat(FinishedSpan actual) {
+        return new SpanAssert(actual);
+    }
+
+    public static SpansAssert assertThat(Collection<FinishedSpan> actual) {
+        return new SpansAssert(actual);
+    }
+
+    public static TracerAssert assertThat(SimpleTracer actual) {
+        return new TracerAssert(actual);
+    }
+
+}