strconv: fix ParseComplex for strings with separators
The recently added function parseFloatPrefix tested the entire
string for correct placement of separators rather than just the
consumed part. The 4-char fix is in readFloat (atof.go:303).
Added more tests. Also added some white space for nicer
grouping of the test cases.
While at it, removed the need for calling testing.Run.
Fixes #38962.
Change-Id: Ifce84f362bb4ede559103f8d535556d3de9325f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/233017 Reviewed-by: Ian Lance Taylor <iant@golang.org>