Add test helper macros. (#465)

- Refactor the existing SNMALLOC_ASSERT and SNMALLOC_CHECK.  These now
   use the FatalErrorBuilder to format the output if a format string is
   provided.
 - Extend the FatalErrorBuilder to print decimal integers for signed
   values.
 - Rename FatalErrorBuilder to MessageBuilder.
 - Rewrite the macros used in the jemalloc tests to use
   FatalErrorBuilder and move them into a header.
 - Refactor some of the tests to use the new macros.

src/test/helpers.h

@@ -0,0 +1,39 @@
+#pragma once
+#ifdef _MSC_VER
+#  define __PRETTY_FUNCTION__ __FUNCSIG__
+#endif
+
+namespace snmalloc
+{
+  /**
+   * The name of the function under test.  This is set in the START_TEST macro
+   * and used for error reporting in EXPECT.
+   */
+  const char* current_test = "";
+
+  /**
+   * Log that the test started.
+   */
+#define START_TEST(msg, ...) \
+  do \
+  { \
+    current_test = __PRETTY_FUNCTION__; \
+    MessageBuilder<1024> mb{"Starting test: " msg "\n", ##__VA_ARGS__}; \
+    fputs(mb.get_message(), stderr); \
+  } while (0)
+
+  /**
+   * An assertion that fires even in debug builds.  Uses the value set by
+   * START_TEST.
+   */
+#define EXPECT(x, msg, ...) \
+  SNMALLOC_CHECK_MSG(x, " in test {} " msg "\n", current_test, ##__VA_ARGS__)
+
+#define INFO(msg, ...) \
+  do \
+  { \
+    MessageBuilder<1024> mb{msg "\n", ##__VA_ARGS__}; \
+    fputs(mb.get_message(), stderr); \
+  } while (0)
+
+}