Compile DEPS files before executing them

Compiling the DEPS files (which are python statements) means that we can
associate a filename with the result code object, which lets `exec`
print the name of the DEPS file in warnings, rather than prefixing with
"<string>:".

This CL also fixes the test to properly initialize a git repository in
the temp dir and pass along the required "--root" flag, otherwise
builddeps.py exits early without actually processing the DEPS file.
(Note that none of the existing assertions looked for _presence_ of any
output; the only assertion verified the _absence_ of banned output.)

Change-Id: I0dbe443398a938b68283cd70c0d888473160f6ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8284611
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Chris Fredrickson <cfredric@chromium.org>
Commit-Queue: Chris Fredrickson <cfredric@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1686387}
NOKEYCHECK=True
GitOrigin-RevId: f683951d6ab98d90f7c688cf4797bc6241871d2f