Add tests for E0755

Author: GuillaumeGomez

src/test/ui/unterminated-comment.rs

@@ -0,0 +1 @@
+/* //~ ERROR E0758

src/test/ui/unterminated-comment.stderr

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+error[E0758]: unterminated block comment
+  --> $DIR/unterminated-comment.rs:1:1
+   |
+LL | /*
+   | ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0758`.

src/test/ui/unterminated-doc-comment.rs

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+/*! //~ ERROR E0758

src/test/ui/unterminated-doc-comment.stderr

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+error[E0758]: unterminated block doc-comment
+  --> $DIR/unterminated-doc-comment.rs:1:1
+   |
+LL | /*!
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0758`.