[superproject] Roll third_party/pigweed pw_string: Reenable insert, erase tests

This CL re-enables tests for insert and erase that were disabled while
debugging CQ failures in pwrev/148333.

These tests push the qemu_gcc version of string_test over the same flash
size threshold that the qemu_clang version crossed some months ago. In
particular, the qemu_gcc_debug version no longer fits into lm3s6965evb's
flash size of 255 KiB. The qemu_clang workaround of only building the
"size_optimized" version of string_test for that target has been applied
to qemu_gcc as well.

Original-Bug: b/244604080
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/149434
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: 7c875693d478ac0da2f358a1542a4fc1cad1b60d
GitOrigin-RevId: b7d92e0dbfad55735ab04a4f6e853909225a249b
Roller-URL: https://ci.chromium.org/b/8779279187908108065
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I7fb4dbf1da06590fdbf3e0b04ae1e31a2bd9a46b
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/865956
diff --git a/third_party/pigweed/src b/third_party/pigweed/src
index ce3e128..7c87569 160000
--- a/third_party/pigweed/src
+++ b/third_party/pigweed/src
@@ -1 +1 @@
-Subproject commit ce3e1280fcac0461258874eea3e917f6b30be573
+Subproject commit 7c875693d478ac0da2f358a1542a4fc1cad1b60d