On Windows, we may omit suffix ".exe" when running an executable.
However, this will trigger an assertion failure in current code and this
CL is to fix this issue.

Bug: angleproject:4640
Change-Id: I7edfdc0b4a7c590c874817530b19a812018b9288
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2206425
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>