runtime/pprof: call runtime.GC twice in memory profile test

[sthagen]

This change fixes golang#46500 by working around golang#45315 which may cause freed
objects to get missed in the heap profile published for the test.

By calling runtime.GC one more time this change ensures that all freed
objects are accounted for.

Fixes golang#46500.

Change-Id: Iedcd0b37dbaffa688b0ff8631a8b79f7a1169634
Reviewed-on: https://go-review.googlesource.com/c/go/+/333549
Trust: Michael Knyszek mknyszek@google.com
Run-TryBot: Michael Knyszek mknyszek@google.com
TryBot-Result: Go Bot gobot@golang.org
Reviewed-by: Austin Clements austin@google.com

This PR will be imported into Gerrit with the title and first
comment (this text) used to generate the subject and body of
the Gerrit change.

Please ensure you adhere to every item in this list.

More info can be found at https://github.com/golang/go/wiki/CommitMessage

• The PR title is formatted as follows: net/http: frob the quux before blarfing
  • The package name goes before the colon
  • The part after the colon uses the verb tense + phrase that completes the blank in,
    "This change modifies Go to ___________"
  • Lowercase verb after the colon
  • No trailing period
  • Keep the title as short as possible. ideally under 76 characters or shorter
• No Markdown
• The first PR comment (this one) is wrapped at 76 characters, unless it's
  really needed (ASCII art, table, or long link)
• If there is a corresponding issue, add either Fixes #1234 or Updates #1234
  (the latter if this is not a complete fix) to this comment
• If referring to a repo other than golang/go you can use the
  owner/repo#issue_number syntax: Fixes golang/tools#1234
• We do not use Signed-off-by lines in Go. Please don't add them.
  Our Gerrit server & GitHub bots enforce CLA compliance instead.
• Delete these instructions once you have read and applied them