reduce flakiness in evict-agent CI

[amoore877]

Pull Request check list

• Commit conforms to CONTRIBUTING.md?
• Proper tests/regressions included?
• [NA] Documentation updated?

Affected functionality
Increase CI reliability

Description of change
Noticed in pushing #5383 that this test would sometimes fail with no explanation. This test is also unrelated to the changes in #5383, so it is flaky. See #5383 (comment) / https://github.com/spiffe/spire/actions/runs/10378374004/job/28734731515?pr=5383#step:9:1301 for unexpected 404 error.

While here, also split up this test into more discrete action units.

Which issue this PR fixes
N/A

[amoore877]

some potential flake here:
https://github.com/spiffe/spire/actions/runs/10459940073/job/28965089994#step:6:66

ok  	github.com/spiffe/spire/pkg/agent/plugin/nodeattestor/sshpop	0.294s
# github.com/google/go-tpm-tools/simulator/internal
In file included from include.c:153:
D:\golang\modcache\github.com\google\go-tpm-tools@v0.4.4\simulator\ms-tpm-20-ref\TPMCmd\tpm\src/crypt/CryptCmac.c: In function 'CryptCmacEnd':
D:\golang\modcache\github.com\google\go-tpm-tools@v0.4.4\simulator\ms-tpm-20-ref\TPMCmd\tpm\src/crypt/CryptCmac.c:164:44: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
  164 |         subkey.t.buffer[subkey.t.size - 1] ^= xorVal;
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
In file included from D:\golang\modcache\github.com\google\go-tpm-tools@v0.4.4\simulator\ms-tpm-20-ref\TPMCmd\tpm\include/Global.h:79,
                 from D:\golang\modcache\github.com\google\go-tpm-tools@v0.4.4\simulator\ms-tpm-20-ref\TPMCmd\tpm\include/Tpm.h:52,
                 from D:\golang\modcache\github.com\google\go-tpm-tools@v0.4.4\simulator\ms-tpm-20-ref\TPMCmd\tpm\src/support/Global.c:57,
                 from include.c:17:
D:\golang\modcache\github.com\google\go-tpm-tools@v0.4.4\simulator\ms-tpm-20-ref\TPMCmd\tpm\include/TpmTypes.h:1350:29: note: at offset -1 into destination object 'buffer' of size 16
 1350 |         BYTE                buffer[MAX_SYM_BLOCK_SIZE];
      |                             ^~~~~~

[azdagron]

thanks @amoore877 !