e2e/NESTED test build in odd location

[dtrudg]

Version of Singularity

main / 4.3

Describe the bug

The e2e suite is building a SIF for the NESTED tests with an odd name, directly in /tmp

TestE2EPARNESTEDSingularity1972956412

Should use the e2e test directory, and be named appropriately.

This is a consequence of using the t.TempDir() function. This is generally seen as good practice in Go. However, in Singularity we try to keep all directly e2e test related files inside a subdir of TMPDIR. This give a nice distinction between files internal to the tests, and things created by the singularity invocations.