runtime/coverage: add missing file close in test support helper
The processPod() helper (invoked by processCoverTestDir, which is in
turn called by _testmain.go) was opening and reading counter data
files, but never closing them. Add a call to close the files after
they have been read.
Fixes #57407.
Change-Id: If9a489f92e4bab72c5b2df8697e14420a6f7b8f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/458835 Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>