Update format string tests to explicitly escape multiple newlines

From what I can tell, the goal of the tests is to ensure that the error
formatting is correct.  I think this is still being tested as intended
after this change.

src/test/ui/fmt/format-string-error-2.rs

@@ -5,7 +5,7 @@ fn main() {
     a");
     //~^ ERROR invalid format string
     format!("{ \
-
+               \
     b");
     //~^ ERROR invalid format string
     format!(r#"{ \
@@ -38,12 +38,12 @@ fn main() {
     { \
     	\
     b \
-
+      \
     ");
     //~^^^ ERROR invalid format string
     format!(r#"
 raw  { \
-
+       \
     c"#);
     //~^^^ ERROR invalid format string
     format!(r#"

src/test/ui/fmt/format-string-error-2.stderr

@@ -19,7 +19,7 @@ error: invalid format string: expected `'}'`, found `'b'`
    |
 LL |     format!("{ \
    |              - because of this opening brace
-LL | 
+LL |                \
 LL |     b");
    |     ^ expected `}` in format string
    |