cmd/file2fuzz: exec main from TestMain instead of running 'go build' in tests
cmd/file2fuzz: exec main from TestMain instead of running 'go build' in tests

This may address a test hang observed in
https://build.golang.org/log/da703ece9e1626eaeabf485e1a3a8180a6bde512.

At the very least, it should make the test much more efficient.

Change-Id: I6a41f9704c8a95276c904f0aee57c4521d642ea7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/390074
Trust: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools