Handle cases where k == 1 in GEMM tester. #3143

Job	Run time
build-and-test / cmake-linux-aarch64	1h 0m 13s
build-and-test / cmake-linux-armhf	1h 0m 13s
build-and-test / cmake-android (arm64)	1m 58s
build-and-test / cmake-linux-riscv64	1h 0m 15s
build-and-test / cmake-android (armv7)	1m 52s
build-and-test / cmake-linux-riscv64-rvv	1h 0m 13s
build-and-test / cmake-linux-x86_64	1h 0m 14s
build-and-test / cmake-android (x86)	2m 7s
build-and-test / cmake-macos-arm64	2m 11s
build-and-test / cmake-windows-arm64	3m 27s
build-and-test / cmake-windows-x64	1h 0m 1s
build-and-test / cmake-windows-x86	1h 0m 5s
build-and-test / bazel-linux-aarch64	6m 28s
build-and-test / bazel-linux-x86_64-clang-18	14m 1s
build-and-test / bazel-linux-x86_64-gcc-9	14m 43s
build-and-test / cmake-ios-arm64	6m 9s
build-and-test / cmake-ios-x86_64	9m 6s
	8h 3m 16s

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