Optimize PR test workflow

[qwang98]

This PR makes 16 GitHub Workflow jobs each with cargo nextest to run on 4 threads (max allowed for ubuntu for GitHub hosted servers), so that each thread can run maximum 1 of the 55 tests.

We are using two workflow runners:

1. One is from provider "warp build", for jobs: build, bench, and run_examples. I assume that this is because these jobs requrie compiling to x86 architecture, which might not be supported by the GitHub Workflow runners? These runners seem to have associated cost with them, so I think we should be careful when optimizing the run_examples job, which is a current bottleneck of 38 minutes. It can be optimized by creating more runners to run the 5 examples in parallel, depending on the timing of each. (Need to calculate associated cost, not sure if it's charged by runner or by running minutes).
2. The other is from GitHub Workflow runner, which seem to have 4 cores for the ubuntu instances that we are using. They are also free for public repos. I'm not sure how many concurrent runners are supported as it's unlimited and free in theory, so I'm allocating 16 concurrent runners for the slow tests. The test_quick job uses 1 runner.

We have 55 slow tests, with the following timing distribution:

PASS [ 21.329s] powdr-pipeline::powdr_std poseidon2_gl_test
PASS [ 25.665s] powdr-pipeline::powdr_std rotate_large_test
PASS [ 4.357s] powdr-pipeline::powdr_std split_bb_test
PASS [ 11.462s] powdr-pipeline::powdr_std split_gl_test
PASS [ 7.930s] powdr-pipeline::powdr_std rotate_small_test
PASS [ 510.098s] powdr-riscv::riscv halt
PASS [ 52.306s] powdr-riscv::riscv many_chunks_dry
PASS [ 637.928s] powdr-riscv::riscv keccak
PASS [ 332.218s] powdr-riscv::riscv vec_median
PASS [ 6.723s] powdr-pipeline::pil block_lookup_or_permutation
PASS [ 19.041s] powdr-pipeline::powdr_std arith256_small_test
PASS [ 471.428s] powdr-pipeline::powdr_std arith256_memory_large_test
PASS [ 3.878s] powdr-pipeline::powdr_std arith_small_test
PASS [ 88.260s] powdr-riscv::riscv output_syscall
PASS [ 564.755s] powdr-pipeline::powdr_std arith_large_test
PASS [ 552.897s] powdr-riscv::riscv std_hello_world
PASS [ 531.372s] powdr-riscv::riscv sum_serde
PASS [ 14.463s] powdr-pipeline::asm dynamic_vadcop
PASS [ 23.791s] powdr-pipeline::powdr_std memory_large_test
PASS [ 19.680s] powdr-pipeline::powdr_std poseidon_bn254_test
PASS [ 373.685s] powdr-riscv::instructions instruction_tests::srli
PASS [ 552.562s] powdr-riscv::riscv double_word
PASS [ 281.095s] powdr-riscv::riscv exported_csv_as_external_witness
PASS [ 255.865s] powdr-riscv::riscv many_chunks
PASS [ 123.384s] powdr-riscv::riscv read_slice
PASS [ 478.133s] powdr-riscv::riscv keccak_powdr
PASS [ 33.031s] powdr-pipeline::powdr_std binary_small_8_test
PASS [ 38.397s] powdr-pipeline::powdr_std poseidon_gl_memory_test
PASS [ 243.517s] powdr-riscv::instructions instruction_tests::bgeu
PASS [ 454.000s] powdr-pipeline::powdr_std keccakf16_test
PASS [ 255.832s] powdr-riscv::instructions instruction_tests::divu
PASS [ 488.605s] powdr-riscv::riscv runtime_ec_add
PASS [ 1.949s] powdr-pipeline::asm vm_to_vm_dynamic_trace_length
PASS [ 5.057s] powdr-pipeline::powdr_std add_sub_small_test
PASS [ 30.576s] powdr-pipeline::asm vm_args
PASS [ 3.776s] powdr-pipeline::powdr_std memory_large_test_parallel_accesses
PASS [ 4.262s] powdr-pipeline::powdr_std memory_large_with_bootloader_write_test
PASS [ 371.261s] powdr-riscv::riscv vec_median_estark_polygon
PASS [ 697.111s] powdr-pipeline::powdr_std keccakf16_memory_test
PASS [ 5.632s] powdr-pipeline::powdr_std memory_small_test
PASS [ 13.504s] powdr-pipeline::powdr_std poseidon2_bb_test
PASS [ 8.284s] powdr-pipeline::powdr_std poseidon_bb_test
PASS [ 7.283s] powdr-pipeline::powdr_std shift_small_test
PASS [ 1.958s] powdr-pipeline::powdr_std split_bn254_test
PASS [ 13.151s] powdr-pipeline::powdr_std reparse::keccakf16_test
PASS [ 368.083s] powdr-riscv::riscv inverse_gl
PASS [ 417.684s] powdr-riscv::riscv runtime_poseidon2_gl
PASS [ 90.563s] powdr-riscv::riscv sum_serde_in_mem
PASS [ 20.122s] powdr-pipeline::pil vm_to_block_dynamic_length
PASS [ 56.939s] powdr-pipeline::powdr_std binary_large_test
PASS [ 42.384s] powdr-pipeline::powdr_std binary_small_test
PASS [ 19.619s] powdr-pipeline::powdr_std shift_large_test
PASS [ 72.619s] powdr-pipeline::powdr_std split_gl_vec_test
PASS [ 246.394s] powdr-riscv::instructions instruction_tests::mulh
PASS [ 437.248s] powdr-pipeline::powdr_std keccakf32_memory_test