TestMapping assumed that there was only one mapping entry corresponding
to /exe/main, but that is not always true.
This CL changes the test logic to examine whether all referenced mappings
are symbolized. Based on the result, the test determines whether the
corresponding mapping entries' HasFunctions fields to be true or false.

I initially attempted to create two mappings for referenced locations
(one for symbolized and another for unsymbolized) as described in the
TODO in proto.go as part of fixing this bug. But that change requires
non-trivial modification in the upstream profile package so I decided
to just fix the test for now.

Fixes #25891

Change-Id: Id27a5b07bb5b59e133755a0f863bf56c0a4f7f2b
Reviewed-on: https://go-review.googlesource.com/119455
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>