idl_parser: Improve error message of pattern match based tests

Made a mismatch error message easier to read.

- Before this CL, if a pattern match fails, an error message was displayed like this:

AssertionError: Mismatched tree at line 5:
Operation() = "foo"VS
Operation(foo)


- After this CL, an error message is displayed like this:

AssertionError: Mismatched tree at line 5:

[EXPECTED]
Operation() = "foo"

[ACTUAL]
Operation(foo)


Change-Id: I6895131d139595b522caf272620aaee473424db3
Reviewed-on: https://chromium-review.googlesource.com/640236
Reviewed-by: Hitoshi Yoshida <[email protected]>
Reviewed-by: Kenichi Ishibashi <[email protected]>
Reviewed-by: Yuki Shiino <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Commit-Queue: Lisa Suzuki <[email protected]>
Cr-Commit-Position: refs/heads/master@{#498052}

• tools/idl_parser/idl_parser_test.py[diff]