Adding tests to prove out prefix, suffix, and %-literal.

- Failing: CatchesRawContentAfterFmt

test/format_test.cxx

@@ -32,3 +32,21 @@ TEST(FormatTest, NCharReturnsNewLine) {
   auto fmt = format::parse_format_string("%n");
   EXPECT_THAT(fmt.process({}), Eq("\n"));
 }
+
+TEST(FormatTest, DoublePctIsLiteral) {
+  using testing::Eq;
+  auto fmt = format::parse_format_string("%%");
+  EXPECT_THAT(fmt.process({}), Eq("%"));
+}
+
+TEST(FormatTest, CatchesRawContentBeforeFmt) {
+  using testing::Eq;
+  auto fmt = format::parse_format_string("TEST%%");
+  EXPECT_THAT(fmt.process({}), Eq("TEST%"));
+}
+
+TEST(FormatTest, CatchesRawContentAfterFmt) {
+  using testing::Eq;
+  auto fmt = format::parse_format_string("%%TEST");
+  EXPECT_THAT(fmt.process({}), Eq("%TEST"));
+}