internal/testdir: simplify and clean up

Now that the bare minimum change to make the run.go test runner into
a normal go test is done, there remain many opportunities to simplify,
modernize and generally clean up the test code.

Of all the opportunities available, this change tries to fit somewhere
between doing "not enough" and "way too much". This ends up including:

• replace verbose flag with testing.Verbose()
• replace custom temporary directory creation, removal, -keep flag
  with testing.T.TempDir
• replace custom code to find the go command with testenv.GoToolPath
• replace many instances of "t.err = err; return" with "return err",
  or with t.Fatal when it's clearly a test infrastructure error
• replace reliance on changing working directory to GOROOT/test to
  computing and using absolute paths
• replace uses of log.Fatal with t.Fatal
• replace uses of deprecated ioutil package with their replacements
• add some missing error checks, use more idiomatic identifier names

For #56844.

Change-Id: I5b301bb83a8e5b64cf211d7f2f4b14d38d48fea0
Reviewed-on: https://go-review.googlesource.com/c/go/+/466155
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Austin Clements <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur