enforce 30 warmup iterations for Base64EncodeDecodeInPlaceTests to make sure it's promoted to Tier 1

[adamsitnik]

@billwert I've continued my [IterationSetup] cleanup and figured out that methods from Base64EncodeDecodeInPlaceTests are executed only once per iteration and once during warmup and they are never promoted to Tier 1.

If we compare the performance for 2 runtimes, we can see a regression:

dotnet run -c Release -f netcoreapp2.2 --filter *Base64DecodeInPlace* --join --runtimes netcoreapp2.2 netcoreapp3.0

Method	Toolchain	NumberOfBytes	Mean
Base64DecodeInPlace	netcoreapp2.2	200000000	168.5 ms
Base64DecodeInPlace	netcoreapp3.0	200000000	460.9 ms

With this change:

WorkloadWarmup   1: 1 op, 461986400.00 ns, 461.9864 ms/op
WorkloadWarmup   2: 1 op, 462728200.00 ns, 462.7282 ms/op
WorkloadWarmup   3: 1 op, 456886200.00 ns, 456.8862 ms/op
WorkloadWarmup   4: 1 op, 458170100.00 ns, 458.1701 ms/op
WorkloadWarmup   5: 1 op, 457636700.00 ns, 457.6367 ms/op
WorkloadWarmup   6: 1 op, 461874100.00 ns, 461.8741 ms/op
WorkloadWarmup   7: 1 op, 456898800.00 ns, 456.8988 ms/op
WorkloadWarmup   8: 1 op, 458063200.00 ns, 458.0632 ms/op
WorkloadWarmup   9: 1 op, 457471300.00 ns, 457.4713 ms/op
WorkloadWarmup  10: 1 op, 458767400.00 ns, 458.7674 ms/op
WorkloadWarmup  11: 1 op, 456662900.00 ns, 456.6629 ms/op
WorkloadWarmup  12: 1 op, 459743400.00 ns, 459.7434 ms/op
WorkloadWarmup  13: 1 op, 458085500.00 ns, 458.0855 ms/op
WorkloadWarmup  14: 1 op, 461621200.00 ns, 461.6212 ms/op
WorkloadWarmup  15: 1 op, 457948400.00 ns, 457.9484 ms/op
WorkloadWarmup  16: 1 op, 456323900.00 ns, 456.3239 ms/op
WorkloadWarmup  17: 1 op, 458770200.00 ns, 458.7702 ms/op
WorkloadWarmup  18: 1 op, 457658800.00 ns, 457.6588 ms/op
WorkloadWarmup  19: 1 op, 457155100.00 ns, 457.1551 ms/op
WorkloadWarmup  20: 1 op, 455714900.00 ns, 455.7149 ms/op
WorkloadWarmup  21: 1 op, 457236900.00 ns, 457.2369 ms/op
WorkloadWarmup  22: 1 op, 460007700.00 ns, 460.0077 ms/op
WorkloadWarmup  23: 1 op, 456098400.00 ns, 456.0984 ms/op
WorkloadWarmup  24: 1 op, 457985500.00 ns, 457.9855 ms/op
WorkloadWarmup  25: 1 op, 461993100.00 ns, 461.9931 ms/op
WorkloadWarmup  26: 1 op, 461049300.00 ns, 461.0493 ms/op
WorkloadWarmup  27: 1 op, 454327700.00 ns, 454.3277 ms/op
WorkloadWarmup  28: 1 op, 455722800.00 ns, 455.7228 ms/op
WorkloadWarmup  29: 1 op, 455981500.00 ns, 455.9815 ms/op
WorkloadWarmup  30: 1 op, 162911100.00 ns, 162.9111 ms/op // got promoted to Tier 1

Method	Toolchain	NumberOfBytes	Mean
Base64DecodeInPlace	netcoreapp2.2	200000000	166.1 ms
Base64DecodeInPlace	netcoreapp3.0	200000000	161.2 ms

[billwert]

@DrewScoggins