strings: TestBuilderAllocs flake on freebsd-386-12_0

[bcmills]

freebsd-386-12_0 (https://build.golang.org/log/380c04df9641b3e5274073af6c25f56e5607e60d):

--- FAIL: TestBuilderAllocs (0.00s)
    builder_test.go:193: got 1 alloc(s); want 0
FAIL
FAIL	strings	1.570s

It's not obvious to me whether this is a bug in the test proper or in testing.AllocsPerRun.

CC @bradfitz; see previously #24647.

[bradfitz]

I see nothing obviously wrong with the test.

Maybe there's a closure allocation that it's counting?

In any case, perhaps we can just delete it. The test at the bottom of the function (which we're not getting to due to the t.Fatal) seems comprehensive at least and more self-contained (no closure either).

[gopherbot]

Change https://golang.org/cl/204598 mentions this issue: strings: remove a redundant and flaky part of a test