Replace the obsolescent fgrep with grep -F in tests

[gthess]

On newer systems I get the message:

fgrep: warning: fgrep is obsolescent; using grep -F

Looking around I see that grep -F is universally supported

• https://man.freebsd.org/cgi/man.cgi?query=grep&sektion=1
• https://man.openbsd.org/grep
• https://man.netbsd.org/grep.1
• https://www.man7.org/linux/man-pages/man1/grep.1.html
• https://ss64.com/osx/grep.html
• https://docs.oracle.com/cd/E88353_01/html/E37839/grep-1.html

[wcawijngaards]

The changes look fine. I could find no problems with it.

[gthess]

Thanks!