[gdb/testsuite] Don't leak env vars in gdb.debuginfod/fetch_src_and_symbols.exp

[gdb/testsuite] Don't leak env vars in gdb.debuginfod/fetch_src_and_symbols.exp

Test-case gdb.debuginfod/fetch_src_and_symbols.exp leaks env vars
DEBUGINFOD_URLS, DEBUGINFOD_TIMEOUT and DEBUGINFOD_CACHE_PATH, causing
timeouts in subsequent tests.

Fix this by using save_vars.  Also, fix PATH and DUPLICATE errors.  Finally,
cleanup whitespace.

gdb/testsuite/ChangeLog:

2020-07-21  Tom de Vries  <tdevries@suse.de>

	* gdb.debuginfod/fetch_src_and_symbols.exp: Use save_vars for env
	vars.  Fix PATH and DUPLICATE errors.  Cleanup whitespace.