Some functions don't have their own tests

[mr-c]

I've added some in https://github.com/nemequ/simde/tree/missing_tests

• [x] _mm256_castpd_ps
• [x] _mm256_castpd_si256
• [x] _mm256_castps128_ps256
• [x] _mm256_castps256_ps128
• [x] _mm256_castps_pd
• [x] _mm256_castps_si256
• [x] _mm256_castsi128_si256
• [x] _mm256_castsi256_pd
• [x] _mm256_castsi256_ps
• [x] _mm256_castsi256_si128
• [x] _mm256_permute2f128_pd
• [x] _mm256_permute2f128_si256
• [x] _mm256_undefined_pd
• [x] _mm256_undefined_ps
• [x] _mm256_undefined_si256
• [x] _mm256_xor_pd
• [x] _mm256_xor_ps
• [x] _mm512_add_epi16
• [x] _mm512_add_epi8
• [x] _mm512_castpd128_pd512
• [x] _mm512_castpd256_pd512
• [x] _mm512_castpd_ps
• [x] _mm512_castpd_si512
• [x] _mm512_castps128_ps512
• [x] _mm512_castps256_ps512
• [x] _mm512_castps_pd
• [x] _mm512_castps_si512
• [x] _mm512_castsi128_si512
• [x] _mm512_castsi256_si512
• [x] _mm512_castsi512_pd
• [x] _mm512_castsi512_ps
• [x] _mm512_set1_epi16
• [x] _mm512_set1_epi32
• [x] _mm512_set1_epi64
• [x] _mm512_set1_epi8
• [x] _mm512_set1_pd
• [x] _mm512_set1_ps
• [x] _mm512_set4_epi32
• [x] _mm512_set4_epi64
• [x] _mm512_set4_pd
• [x] _mm512_set4_ps
• [x] _mm512_set_epi16
• [x] _mm512_set_epi32
• [x] _mm512_set_epi64
• [x] _mm512_set_epi8
• [x] _mm512_set_pd
• [x] _mm512_set_ps
• [x] _mm512_setr4_epi32
• [x] _mm512_setr4_epi64
• [x] _mm512_setr4_pd
• [x] _mm512_setr4_ps
• [x] _mm512_setr_epi32
• [x] _mm512_setr_epi64
• [x] _mm512_setr_pd
• [x] _mm512_setr_ps
• [x] _mm512_setzero_pd
• [x] _mm512_setzero_ps
• [ ] _mm_castpd_si128
• [x] _mm_castps_pd
• [ ] _mm_castps_si128
• [x] _mm_set1_epi16
• [x] _mm_set1_epi32
• [x] _mm_set_epi16
• [x] _mm_set_epi32
• [x] _mm_set_epi64x
• [x] _mm_set_epi8
• [x] _mm_set_pd
• [x] _mm_set_pd1
• [ ] _mm_set_pi16
• [ ] _mm_set_pi32
• [ ] _mm_set_pi8
• [x] _mm_set_ps1
• [x] _mm_setr_epi64
• [x] _mm_setr_pd
• [x] _mm_setzero_pd
• [x] _mm_setzero_si64
• [x] _mm_store1_pd
• [x] _mm_undefined_pd
• [x] _mm_undefined_si128

The following are missing tests, but I believe they are unsupported / no-ops for SIMDe:

• _mm_clflush
• _mm_empty
• _mm_getcsr
• _mm_lfence
• _mm_mfence
• _mm_pause
• _mm_prefetch
• _mm_setcsr
• _mm_sfence

_mm256_alignr_epi8 Initially showed up on my list because it is tested via multiples tests: test_simde_mm256_alignr_epi8_case0 throught test_simde_mm256_alignr_epi8_case3