BENCH: FromDictwithTimestampOffset benchmark is not useful
[Reference](https://github.com/pandas-dev/pandas/pull/18499#issuecomment-346981337) 

In the `frame_ctor.py` benchmark, `FromDictwithTimestampOffset` which perform a lot of setup just to benchmark a dict of Timestamp keys (with an offset) and float values. Given that it's the longest benchmark to run (xref #16803), I'd be great if this benchmark can be cut down to just test one offset. 

For example, contructing a DataFrame from this dict of Timestamps with offsets/floats is pretty constant regardless of offset at n=100.

```
asv dev -b ^frame_ctor.FromDictwithTimestampOffsets
· Discovering benchmarks
· Running 1 total benchmarks (1 commits * 1 environments * 1 benchmarks)
[  0.00%] ·· Building for existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  0.00%] ·· Benchmarking existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[100.00%] ··· Running frame_ctor.FromDictwithTimestampOffsets.time_frame_ctor                                                         ok
[100.00%] ···· 
               ==================== ======== ========
               --                        n_steps     
               -------------------- -----------------
                      offset           1        2    
               ==================== ======== ========
                       Day           11.6ms   11.8ms 
                   BusinessDay       11.6ms   11.6ms 
                       BDay          11.6ms   11.6ms 
                CustomBusinessDay    11.6ms   11.6ms 
                       CDay          11.6ms   11.7ms 
                    CBMonthEnd       11.6ms   11.7ms 
                   CBMonthBegin      11.7ms   11.5ms 
                    MonthBegin       11.5ms   11.7ms 
                   BMonthBegin       11.5ms   11.5ms 
                     MonthEnd        11.8ms   11.5ms 
                    BMonthEnd        11.5ms   11.7ms 
                   SemiMonthEnd      11.6ms   11.5ms 
                  SemiMonthBegin     11.6ms   11.6ms 
                   BusinessHour      11.8ms   11.7ms 
                CustomBusinessHour   11.6ms   11.6ms 
                    YearBegin        11.4ms   11.8ms 
                    BYearBegin       11.5ms   11.6ms 
                     YearEnd         11.5ms   11.5ms 
                     BYearEnd        11.8ms   11.6ms 
                   QuarterBegin      11.5ms   11.5ms 
                  BQuarterBegin      11.5ms   11.6ms 
                    QuarterEnd       11.6ms   11.7ms 
                   BQuarterEnd       11.6ms   11.9ms 
                 LastWeekOfMonth     11.6ms   11.8ms 
                       Week          11.6ms   11.7ms 
                   WeekOfMonth       11.6ms   11.5ms 
                      Easter         11.6ms   11.6ms 
                       Hour          11.5ms   11.4ms 
                      Minute         11.6ms   11.7ms 
                      Second         11.7ms   11.5ms 
                      Milli          11.6ms   11.6ms 
                      Micro          11.4ms   11.4ms 
                       Nano          9.33ms   9.23ms 
                    DateOffset       11.7ms   11.8ms 
                     FY5253_1        11.6ms   11.5ms 
                     FY5253_2        11.8ms   11.7ms 
                 FY5253Quarter_1     11.7ms   11.7ms 
                 FY5253Quarter_2     11.7ms   12.2ms 
               ==================== ======== ========
```